Time ran more slowly then because there were no accurate clocks back then. Plus, Roman historians were notorious liars. I'll bet motivations were greater back then, especially with the Romans being so found of crucifixion, decimation and flogging.
I can't seem to find it, but I believe some of our more elite US forces can do ruck marches that are pretty close to the Roman numbers. It's a hell of a fast pace, though and type of terrain and footwear makes a big difference. Here's one non-official site:
-- Bobby G.