Humans haven't changed much physically, and soldiers in well-regarded armies train as hard now as they ever had. It's just the type of fitness that has changed throughout history a bit based on the changing physical requirements of soldiers. Roman infantry would have had superior upper body endurance and perhaps strength due to the great emphasis on their hand to hand combat training with sword and shield. And javelin throwing. On the other hand, well trained modern soldiers would be better at crawling because they do it so much. Legionaries did more building and manual labour than modern soldiers too so in general they were probably a bit stronger in the upper body for their size.

Marching has been a constant element in military training and operations ever since Philip II turned his phalanx into a serious campaigning army. Romans could march, build and fight with the best of them, but I see no reason why they would have been any better at it than any experienced pre-modern or modern army. Romans did more road marching than modern soldiers but they didn't bushwhack as much: I'd put my money on a legion on the road, but a modern elite light infantryman would be about as physically suited for running or patrolling cross country as soldiers ever have been.

What Romans did have is a huge training advantage over their contemporary enemies, who generally did not have the training, logistics or organisation to march and operate over large areas for extended periods.