Coconut,
Believe it or not, I'm not very "cut" or big (I'm 5'11 180 pounds, about a 32" waist). 100 pushups a day is nothing. I usually do it in two sets of 50. Sometimes I'll do the straight 100, but usually 75/25 or 60/40 or 50/50. Some days I'll up to 200-300, but rarely. It's just a ritual that I do (make it a point to get in at least 100. Sometimes I don't do it for whatever reason).
I ran the two marathons in 2002. I didn't do as well as I would have liked. I ran the first one too fast in the beginning and finished in 3hours 58 minutes. I was hurting the last 5 miles! The second one I didn't train as hard and ran easy in the first half marathon and beat my time by 20 minutes! I run the half marathons much faster pace (like most people), around 7:25-7:45/min. miles. I'd like to do one more while I physically can!
Right now I've been doing about 15-30 miles per week. I can do a 5K (3.1 miles) in about 6:30/min miles. Once I get over 5 miles the pace is about 7:00-7:15/min miles. Over 10 miles the pace gets slower (obviously). It's getting harder as I get older!
As far as the pull-ups and "burning out". This is different w/ every person, but when I do pull-ups I get weaker each set. I can jump up and do 12-15 pretty solid. But the next set I'd only get about 8-10. That's why I said I could do three sets of 10, but not 12. If I did 12 the first set my back (lat muscles) would get worn out/burned out quickly. Some people are different. I've seen people go until they couldn't do one more....then 2-3 minutes later they are doing the same number they did the first set. Amazing!