NEW YORK, Nov 5 - Seven-times Tour de France champion Lance Armstrong reached his goal of beating the three-hour mark by finishing the New York City Marathon in two hours 59 minutes and 36 seconds on Sunday.

Lance Armstrong runs in the New York City marathon in the Brooklyn Borough of New York. [Reuters]

Armstrong said the marathon was tough work.

"Without a doubt that was the hardest physical thing I've ever done," said the 35-year-old, noticeably limping.

Armstrong, running his first marathon as a personal challenge and to benefit his foundation and raise money for cancer research, was cheered all along the route through the city's five boroughs.

"Certainly without the support of New York City I would have been three and a half hours," said Armstrong, who was surrounded by Team Armstrong runners who helped raise more than $600,000.

He was also helped by pacesetters including former race winner Alberto Salazar, former Olympic champion Joan Benoit Samuelson and Olympic 1,500 and 5,000-metre winner Hicham El Guerrouj.

Armstrong trained about 45 minutes to an hour a day in preparation for the marathon with his longest pre-race run lasting 16 miles.

"It was always a goal of mine to do a marathon," he said. "I figured in retirement, running would be a much more logical mode of exercise than racing because I'm on the road so much and running is simply easier to do with travel.

"I suppose I didn't train sufficiently enough for a marathon."

Armstrong weighed in at about 180 pounds for the race, a gain of nearly 20 pounds over his riding weight.

"You have to remember that the last seven years of my career I was in the best possible shape. It was my job. I was literally paid to be a winner. And so I took it very seriously and I focused daily. It was all that I did," he said.

"I can tell you, 20 years of pro sports, endurance sports, from triathlons to cycling, all of the Tours, even the worst days on the Tours, nothing was as hard as that and nothing left me feeling the way that I feel now in terms of just sheer fatigue and soreness."

Armstrong was not sure about trying another marathon.

"The answer right now is no, I'll never be back," he said, drawing laughter from reporters at his news confrerence. "But I reserve the right to change my mind."