I agree with tribro, and I want to add..

consider that everyone has different abilities and paces.
- running 2 hrs for you might mean going 12 miles, but I might manage only 10 miles in that time.

Nonetheless, your body will react to a 2 hour run much the same way as mine does, because we will burn up our reserves of fuel in a given time regardless of our ability. Which is to say, I will get as much out of a 2 hour training run as you do relative to my lower ability.

20 minutes of exercise and we both run out of glycogen and begin to metabolize more fat for fuel.

After 2 1/2 hours you will have exhausted all fuel reserves and, unless you have been eating something during that period the body will begin to catabolize muscle for energy, (as well as using fat reserves and reconstituted glycogen)

- even though we will have covered different distances during that time.

Yeah, covering the distance on race day is the goal, but managing training periods, especially to avoid doing "junk miles," one needs to address different parameters.

best of luck and keep those questions coming,

PoC

It has always seemed a bit odd to me as well, and I came to the same conclusions as already posted. However, I wanted to add good luck!

PGans,

POC and Tribro make important points. I would add, it is critical to consider your own abilites. If you are a 10 min. miler, then you may want to consider clocking off some long run courses. This will ensure you cover an appropriate distance to be prepared.

In my own training, I use both methods. For long rides and runs, I always know how far I've gone not just the elapsed time. I'll set on the bike and say I'm going to ride for six hours, but I'm also concerned with distance. Some of it is mental, you want make sure you are ready to see 90 miles on the computer and know that you have another hour + on the course.