Maybe the difference is conditioning, rather than fitness. Maybe its just semantics like, but players today are better conditioned to go like Bilio for 90 minutes, much more than in the 70s, say. As Brazilianbob says, they are so fine tuned that the muscles are (particularly hamstrings I think) are more likely to go.
There is no doubt that the game is faster these days, but that doesn't mean that there weren't individual footballers in the 70s (Colin Bell to use use your example) who weren't as fast if not faster than todays players.
I read a good interview with Socrates in one pf the papers last year, where he said that football is less enjoyable to watch (for him) precisely because it is much faster and the players run more and faster. If the players run more and faster, but the pitches are the same size, therefore there is less space available... which means its much more difficult to play the ball into space, and therefore more difficult to come up with a real defence splitting pass, which for him used to be oine of the most beautiful things to see in a football match.
The other side of it is, as others have said, that we will never know if the great players form the past would be just as great now, with the same conditioning that the players today benefit from. What I would say is that if 1986 Maradona was transported into any current league in the world I'd bet a lot of money that he would still be head and shoulders above anyone else, easily, even with the faster game. And if he had been training everyday at one of the top modern clubs like Arsenal or Man United, the mind boggles about how good he might have been. Maybe that's not a fair example though, cos he was a real one off. You can use the same logic for the likes of Green, Bell et al, but to a lesser extent.