The fitness requirements and training regimes of Romans compared to an active Western army were different but I think quite easily compared. The Romans did a lot of weapons practice (sword and shield, javelin throwing), a lot of manual labour, and a lot of marching. Modern soldiers do a lot of running, marching, obstacle courses/dynamic/weighted PT, crawling, climbing, all sorts of stuff.

The core duties of modern infantry is patrolling and attacking, which involves a lot of heavily weighted walking, running, crawling and depending on the environment may also involve climbing, all while wielding weapons, a fair bit of equipment and usually wearing armour. The core duties of legionaries were what their training entailed: hand to hand fighting, marching, and manual labour. Much simpler and can be practiced every day, so they were very good at it.

However modern soldiers have the support of more systematic and organised training regimes than did the Romans, as well as better diet and science to back it all up. I don't think that the Romans would have trained any harder than a good modern infantry unit. The Romans were amazing at marching long distances and could build and fight at the end of it, but modern soldiers can be incredible marchers too: look at the German infantry in 1914 executing the Schleiffen Plan, or invading Russia in 1941. I don't see any reason why modern soldiers who have trained a lot for road marching should be much worse if any worse than the average legionary.

In terms of upper body strength, modern soldiers do a lot of climbing, push ups and stuff. But the Romans trained potentially every day with sword-and-board, and especially on campaign their daily duties entailed significant manual labour. I'd say the Romans would have better upper body endurance due to their different job requirements, but due to things like diet and physical size I wouldn't say they'd necessarily be any stronger.

Overall, soldiers have to train realistically to be good at what they do, no matter what the period. Romans would be better at road marching all day then cutting down half a forest at the end of it. Modern infantry would be better at humping up and down hills with heavy packs and rifles/guns all day and night. Modern infantry would crawl further and faster than legionaries, but legionaries could stand toe to toe with a massive Gaul and kill him with a sword. Just different jobs really. But the things they both did, especially road-marching, I think they would perform somewhat similarly.*

• I talk a bit about marching in comparative modern and ancient contexts here

EDIT: Sorry, I didn't mention WWII GI's. This all applies, and I daresay they actually did more road-marching than the average modern soldier. Dedicated infantry recruits, as opposed to replacements pulled from rear areas when the going got tough, did a lot of training State-side and were very fit when they deployed. I daresay probably could have kept up with a legionary on the road. The rest of my post stands without modification I think.