I've always felt that how quickly I recover from hard workouts is the best indicator of what kind of shape I'm in.
I ran 30 miles on Saturday. It was a pretty good run. I had a stretch in the middle (from about mile 13-20) in which I felt pretty horrible but before and after that I felt pretty good.
I then got up at 7:00 on Sunday to go out for 20 miles. I was pretty tired by the end of that... but mostly just hungry. I came home and rested for 4 hours, eating about 2,000 calories in that time. At 2 pm I headed back out for 10 miles as well as some time at the gym. The amazing thing was that I felt great in this afternoon session. Almost no fatigue and my legs felt very fresh and quick. After that I went to work (on my feet) for 5 hours and still didn't feel tired out much at all when I got home at 10:30 pm.
I had run 60 miles in 28 hours and felt 100% recovered before going to bed. This morning I feel great. No fatigue. No soreness. I basically feel like I took a couple days off.
Apparently my body has learned to recover from 4+ hours of running with just a few hours of resting, hydrating, and eating. It's a good feeling to be 6 weeks away from the toughest physical challenge of my life and know that I am in the best shape I have ever been in.