Conditioning is one thing, but I don’t think that Marines would be less fit. Diets are so, so much better. I’d be more worried about mental fitness. Much of the Marine doctrine is to be a part of the whole system. Cut off from that system would be very hard to adapt.
I am not sure the Roman soldier’s diet was inferior. They ate mostly whole grains. When supply lines weren’t interrupted they seem to have been remarkably healthy. At least you seldom read about sickness being a big problem, even during sieges, whereas during the medieval and early modern periods it was always a race between starvation of those in the fort and the besieging army killing itself from filth.
The Romans may not have understood the germ theory of disease, but they were really good at practical sanitation. We didn’t climb back to their level till the middle of the 19th century.