"He has been out doing marathons and riding a lot of long-distance charity rides, so he's had to stay in pretty good shape to do those things on a regular basis.
"When he sets his mind to doing something he gets it done. He obviously believes he can win again. He doesn't make these kind of decisions idly."
Italian Davide Rebellin agreed that Armstrong would be in the best possible shape when he eventually returns to competitive racing.
"From the moment he starts training, he'll be on his way to competing," said the 37-year-old Gerolsteiner rider.
"I know how him well and how he is. He'll do all he needs to get the results. If he returns, he'll surely be competitive."
Another former team-mate, Sean Yates, believes the close bonds riders form with other cyclists would have been one of the key factors in his return.
"He has made it clear that he misses the camaraderie of the bike team," the 48-year-old Briton told BBC Radio 5 Live.
"When you're part of a team your part of a close-knit group."
However, there are several people who are not convinced Armstrong has made the right decision.
"As a cyclist, I don't understand it," said Spaniard Alejandro Valverde. "But everybody is their own person and can do what they want.
Armstrong has kept himself in good shape since retiring in 2005
"It seems surprising to me, after having left cycling and even more so after having won seven Tours, but if he is fit and hasn't lost his form then he will be welcomed back."
Carlos Sastre, this year's Tour de France winner, added: "I imagine that he's studied everything and he will have looked at the pros and cons. If he's done that, then it's fine with me."
Speaking before Armstrong's return was made official, Britain's David Millar said: "It seems a bit ambitious.
"I am very surprised because I was fairly sure he had put cycling behind him and he was moving on with his life."
But the Scot admitted Armstrong, who will shed more light on his comeback plans on 24 September, is no ordinary cyclist.
"Lance has proven in the past he can do almost anything," said Millar. "It sounds totally off the wall at first, but that's the sort of person Lance is."
Asked what kind of reaction Armstrong's comeback had sparked amongst his fellow cyclists, he added: "Surprise more than anything. Curiosity as well to see what he can do and what he can't do."
Armstrong has been plagued by accusations of doping through his career, although he has never failed a drugs test and maintains he is a clean athlete.
He's been a superb athlete, both in coming back from cancer and in winning seven Tours
UCI president Pat McQuaid
And he has voiced fears that race organisers, the Amaury Sport Organisation (ASO), could bar him from competing again as part of their crackdown on doping.
But race director Christian Prudhomme said the Texan will be able to compete if he fully submits to the latest anti-doping controls.
"As long as his team, which we don't know for the moment, and he himself abide by the rules concerning doping and anti-doping which have considerably evolved in the last few years, we will accept him," said Prudhomme.
"But under no circumstances will we accept a cyclist who refuses to abide by these rules which are stricter and of a different nature than previously."
Pat McQuaid, president of the International Cycling Union (UCI), added that Armstrong was "free to race".
"There's nothing to stop him coming back," he said. "There is no administrative, legal or sporting issue to stop him.
"From the UCI's point of view, he's free to race.
"He can come back, but the question is if he can return to the same level. Maybe he doesn't know that himself, maybe he just wants to see what he can do.
"He's been a superb athlete, both in coming back from cancer and in winning seven Tours."
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