In my case, my >100ft/mile rides average about 5 mph slower that my relatively flat rides. This is both because I get a lot slower when climbing steep hills, and I am a wuss on the descents. But if I use this as a crude estimate, 500 miles of up/down = 750 miles of flat.

Someone mentioned if you are riding in a closed loop, you have no net elevation gain and therefore, implicitly, nothing that differs from a flat ride. That isn't true, because a major component is overcoming gravitational potential energy (mgh, m = mass of rider+bike, g is the gravitational constant, and h is the vertical height climbed). It takes calories to surmount that potential energy barrier, but you don't regain those calories when you coast down the hill. Same thing with friction (tires on the road, wind resistance, etc).