The elevation is one thing, for sure. If you burn all your matches early, it's bad news in the long run.

The thing I learned too, was that in rides over about 40 miles, fueling and hydration become a big deal. In shorter rides, if you're well-hydrated prior to the ride, you can compensate and recover later. If you're burning so many calories and sweating so much though, it's too late not to do more during the ride.

On my first century, I bonked at 80 miles, had to rest for awhile while my body re-grouped, then pedaled the last 20 very slowly. (luckily it was flat)

I did manage to stay hydrated, but I burned off the bacon cheeseburger, fries and two beers sooner than I thought I would and just plain ran out of energy.