WASHINGTON — Under blue skies on a blazing hot day this summer, John Kerry hopped onto his bike, clipped into his pedals, and spent three hours on a grueling ride up a mountain in the Alps.

This was a year after he fell off his bike and broke his femur. This was after days in the hospital and months of physical therapy. This was after he became the butt of jokes from Donald Trump.

Now he was back on the bike, trying to ride the same Alpine pass, featured in the Tour de France, that he’d failed to complete a year earlier.

“I wanted to go back,’’ Kerry said. “I just was not satisfied. . . . I said, ‘I’ve got to go pick up where I left off.’ ’’

It has sometimes seemed, too, that Kerry as secretary of state has tried to change the world’s ills through sheer determination and stamina. And in some important cases — like the deal to limit Iran’s capacity to create a nuclear weapon, or helping persuade nearly 200 nations to sign onto a climate agreement — his tireless efforts yielded historic results.

But, as his critics have noted, not all the world’s problems can succumb to individual will. A case in point is the future of civilians in Aleppo, the most tragic result of America’s inability to change the trajectory of Syria’s civil war, hangs heavily on Kerry’s mind.

Even as he recited some of the “what-ifs’’ on Syria policy and internal Obama administration disagreements, Kerry worked the phones with various foreign ministers last week in a bid to provide a safe haven for Aleppo’s people, who were being killed by the Syrian military’s final push to seize the city.

“We’re just trying to find a way to stop the violence,’’ he said. “And it’s very difficult.’’

Will return to Boston

Kerry, during a wide-ranging interview with the Globe, reflected on Syria, Israel, Iran, and other crisis spots in American foreign policy as he prepares to step down next month from the post he has held since 2013.

The former Massachusetts senator plans to return to Boston. He said he will seek work in the private sector, declining to provide details while he still holds his public job. And he said he looks forward to shedding the diplomatic reserve that has prevented him from speaking out on domestic politics over the last four years.

As Kerry sat in his State Department office, he was nursing a cold, sucking on cough drops and drinking vitamin water — a key source of nutrients for a man with a frenzied, peripatetic schedule.

“You hydrate. When you’re flying around and changing time zones, that’s the secret. Stay hydrated,’’ Kerry said. “People don’t drink enough water. Really. . . . That’s when you get tired and lose track of where the hell you are.’’

On election night, the 2004 Democratic presidential nominee was about as far away from American politics as he could get: He was en route to Antarctica, to observe scientists studying the impacts of climate change.

Aboard his 757 government plane, he followed the results and quickly began to realize that Hillary Clinton was going to lose in one of the biggest political upsets in modern history.

“I called friends from the plane and I said, ‘This is bad. It’s looking over,’ ’’ Kerry said. “You could just tell when you saw the numbers in Pennsylvania and Wisconsin and Michigan. I thought the pattern was pretty clear. . . . The handwriting was on the wall. It was hard. It was a difficult thing.’’

Kerry said that at one point he thought about getting into the race himself.

“For a minute or two, maybe somewhere along the line, it crossed my mind — possibly,’’ he said. “But there are a number of reasons why I dismissed the idea. And I’m glad. I think it was right. I never really thought that seriously about it.’’

He declined to say when he had these thoughts.

“I’m not going to go into that. . . . It passed my mind a few times where I thought the issues were such and the campaign was — sort of seemed complicated,’’ he said. “But I stayed away. I didn’t get into any serious conversation. Mostly because I love this job, and I really said, ‘I want to finish this.’ ’’

“As any former recovering politician, your juices get going. You work for as long as I have, 1982, ’81, in competitive-slash-electoral politics. There’s a time of year when the sky changes, the wind changes, the temperature changes. You get into the fall. . . . You feel it. Everybody who’s involved in it says, ‘Whoa. It’s that time,’ and the juices flow.’’

In public office 34 years

Kerry is concluding not just four years as secretary of state, but nearly 34 years in public office.

Yet as he reflects on his highest-profile job, he is also facing a stark reality: many of his major accomplishments may soon be shred to pieces. Trump has railed against the Iran deal that Kerry spent years negotiating, and he has pilloried the Paris climate deal. Trump has threatened to reinstate sanctions on Cuba, and could revoke them on Russia.

Kerry remains diplomatic when questioned about Trump’s plans. “There are a lot of things that get talked about in the course of a campaign,’’ he said. “How many people have run for office saying I’m going to do this and that?’’

Kerry believes the Iran deal “will stand on its rationale.’’ He praised as thoughtful some of Trump’s nominees, including defense secretary nominee James Mattis and Rex Tillerson, who Trump has selected to be Kerry’s successor.

“Let’s wait and see. I’m not going to get ahead of the decisions that they make,’’ he said. “Because governing is very, very different when you’re confronted with the realities. And I think one of the things people make a huge mistake in doing is ginning themselves up over guessing. I’m not going to guess.’’

One of the conflicts that has preoccupied Kerry during his tenure at the State Department has been the devastation in Syria, with a death toll that, by some estimates, has approached 500,000.

In 2013, after a chemical weapons attack by Syrian President Bashar Assad’s forces on civilians, Kerry took the lead in the Obama administration in advocating a forceful response, delivering remarks in vivid and emotional terms. But he lost an internal battle to push for military strikes.

He later negotiated the removal of chemical weapons from Syria, but the civil war has continued to rage.

More recently, he has clashed inside the administration with Defense Secretary Ashton Carter. Kerry negotiated an agreement with Russia to share joint military operations, but it fell apart.

“Unfortunately we had divisions within our own ranks that made the implementation of that extremely hard to accomplish,’’ Kerry said. “But I believe in it, I think it can work, could have worked.’’

“It’s late now obviously because of what’s happened to Aleppo,’’ he said. “But the fact is we had an agreement in which Russia gave us a veto over their flights and over what they were doing in the area, had we set up a joint cooperative effort.’’

“Now we had people in our government who were bitterly opposed to doing that,’’ he said. “I regret that. I think that was a mistake. I think you’d have a different situation there conceivably now if we’d been able to do that.’’

Now, the country is in turmoil, with reports of women and children being slaughtered as they try to flee Aleppo. There are reports of executions, with desperate Syrians pleading for help on social media.

Kerry has grown increasingly frustrated.

“The Russians and Assad are basically throwing everybody into the same pot and acting way outside of any decent standard of warfare . . . and challenging all of the norms of expected humanitarian behavior.’’

Kerry has been pushing to establish a humanitarian corridor, and a United Nations monitoring system to help people flee. He is also hoping to establish a cease-fire and then join for talks in Geneva.

“That’s the only ultimate solution to the war,’’ Kerry said. “The issue is, are you going to be able to patch the country back together.

“I’m trying to line it up so the opposition will understand and embrace the idea of having those talks, even at this difficult moment. And it’s hard.’’

Kerry’s tenure has been marked by diplomatic risk-taking and a crushing schedule. He has logged 1.4 million miles, more than any secretary of state in history.

He’s spent the equivalent of 124 days in the air. The schedule is so extreme — and unpredictable — that members of his press corps have called traveling with him the “Kerry Go-Round.’’

“The last four years, I think we’ve been as productive as any secretary in any administration in history,’’ Kerry said.

“Some people try to assert a narrative that somehow this administration was disengaging or pulling back. . . . But the truth is, and the facts support it, that we are — the United States of America — more deeply engaged on more issues of significance, all simultaneously, with consequence, with results, than at any time in the history of our country,’’ he said.

Kerry’s signature accomplishment was negotiating a nuclear deal with Iran, the result of years of diplomacy. It marked the first time in decades the two countries had any diplomatic relationship, and it resulted in a deal that lifted economic sanctions on Iran in return for a suspension of its nuclear program.

“It’s in place and it’s working,’’ Kerry said. “And Iran’s nuclear program which was rushing towards a bomb and rushing towards conflict as a result is now a program from which you cannot make a nuclear bomb.’’

Kerry also helped negotiate an international agreement to curb greenhouse gas emissions, and he has started an annual conference on improving the quality of the oceans.

Kerry also brought up the Trans-Pacific Partnership trade deal, which he helped negotiate. The deal stalled in Congress, and Trump is an adamant opponent. Despite all odds, Kerry maintains that it could still be resurrected in some form.

“Obviously it’s not going to move right now, but I predict it will ultimately,’’ Kerry said. “Because we cannot lose this notion of America helping to raise the standards of doing business on a global basis. “

Few personal regrets

Kerry said he has few regrets. “I don’t feel them as a personal regret, ‘Oh my gosh I didn’t do something or I didn’t try hard enough,’ ’’ he said. “Some things are just not ripe in diplomacy.’’

He spent much of his first year in office attempting to get the Israelis and Palestinians to come to an agreement. Some of his critics said Kerry was wasting time, that his inveterate optimism was never going to change minds. Kerry doesn’t see it that way.

“If the two leaders do not want to take a risk and they do not want to move in a direction, nothing you can do,’’ Kerry said. “No matter how sensible or palatable your proposal might be.’’

Kerry has been worried about the state of American politics for some time, decrying especially the degraded discourse in the Senate, where he spent much of his career. Yet he remains proud of the involvement of the United States in the world, and feels his tenure has shown the value of engagement and diplomacy.

“People don’t say where’s China, where’s Russia, where’s whatever country. But they do say, and expect, the United States of America to be there on these kinds of things,’’ he said. “We are expected to lead. And I hate the way in which we are shooting ourselves in the foot with our politics, with the inability of Congress to do its budgets, with the inability to try and send a message about the real strength of democracy.’’

“It’s very hard to walk into a country and look at the prime minister or the finance minister and you urge them to do their budget, or urge them to get more democratic. ‘You mean like your last round?’ They laugh at you.’’

Not ready for retirement

Kerry just turned 73, and while he’s had both hips replaced it hasn’t prevented him biking up mountains with people who are three decades younger. Some who are Kerry’s age might consider retirement, or at least an extended vacation. He is not.

He will make Boston his home base, but is also planning to work in Washington. He has been contemplating a range of things that will occupy him once he formally leaves office at noon on Jan. 20.

He wants to write about his experiences. He’s been talking to environmental groups about being engaged in continuing to fight climate change.

“No question I’ll be speaking out and engaged politically in the debate of our country,’’ Kerry said. “You can imagine, it was very hard to sit there during the presidential race and bite your tongue and lip and stay silent. As of Jan. 20, I don’t have to do that anymore.’’

He also intends to work in the private sector for one of the first times since he opened a cookie shop in 1976 in Quincy Market.

“I’m going to continue to work for peace and conflict resolution in a constructive forum that I’m trying to think about and shape right now,’’ he said. “I want to do private sector.’’ He would not elaborate much on what that might entail.

He’ll spend time with his grandchildren — one of whom sat in his lap while he signed the Paris climate deal. A few bike rides could be in his future.

Nearby, inside Kerry’s private office sits a model of the swift boat that he commanded in Vietnam.

And as he sat and thought about what’s next for him, his mind turned to his Vietnam buddies, some of whom never came home.

“We had a saying,’’ Kerry said, “that every day is extra.’’

Matt Viser can be reached at matt.viser@globe.com.