I would consider the Inca Bridge trail to be relatively-level. Some steps in the beginning but nothing too hard. Nothing compared to Huanapicchu (which I also did). It's the steepness at elevation that made that one difficult. I'd spent the previous 4 weeks at or close to sea level (22 day cruise from Vancouver to Chile) so had no altitude conditioning at all (not that I have any where in my range).

Nothing to do for altitude other than living there for at least 2 weeks.

I also did the walk out to the Gate of the Sun and while that was level, it was also quite rough.

FWIW, I'd recommend heavyweight boots or those who sturdy soles as you'll feel every single, uneven rock that the Incans paved with light-weight hikers.