Originally Posted by
Drew Eckhardt
Time. Two hours at a given intensity is the same whether you cover 40 miles on flat ground or 20 miles with 5000 vertical feet up.
"At a given intensity" being the key phrase. 20 miles with 5K feet is going to be way more intense than cruising at 20mph for two hours on the flats unless you're talking about a gradual 5K climb rather than a ride that ends where it starts.
Steep hills can require a level of intensity that you wouldn't want to use on flats for the simple reason that you'll burn your muscles out prematurely.