Hmm, I think this pushup program suffers from the same problem than some of the similar ones I've tried in the past. The progression is fine initially but then you hit a certain plateau past which it's really hard to get. Here's another pushup routine, which may be methodologically better:
http://www.impulseadventure.com/weights/100push.html Part of the problem may be related to one's physical build as well. I used to be very active in the past (boxing, martial arts) but as a tall, fairly lightly built guy I could never get past about 50 pushups. Some problem with the bench press though I've been much better on some other lifts. In my experience guys with shorter arms can peform such exercises more easily because the leverage is different.
But anyway, I also gave this program a shot. Since I don't work out as hard as I used to it seemed a good idea. But I can tell right now after just a week into the routine (I started with Week 3) that I won't be ready for 100 reps any time soon. Anyway, even just getting back to 50 would be plenty nice for starters...