I think one important factor in getting into peak fitness is setting goals. In the past, I was always in much better shape when I was training for competition. It's always harder to make yourself train hard when there's not an upcoming race or game or competition of some sort. The exception to this was weightlifting. I always liked weightlifting and never competed in it. But with lifting, the benchmarks were clear. If I benched 185lbs 10 times last month, I knew I was improving if I could do 195lbs this month.
One of the great things about Crossfit is that there are many, many benchmarks for measuring your progress. Most of them are short, custom-designed workouts that bear girls' names, like "Angie" (100 pull-ups, 100 push-ups, 100 sit-ups, 100 air-squats , for time), "Diane" (21+15+9 reps of 225lb deadlifts and handstand push-ups, for time), or "Nancy" (5 rounds of 400 meter run, 15 overhead squats at 95lbs, for time).
[The fact that most of the benchmark workouts have girls' names also makes it fun because you can say things like, "I did Fran last night in five minutes and thirty-two seconds."]
But some of the relevant benchmarks are much simpler. Like pull-ups. When I started Crossfit in early December, I could do about 2 legit pull-ups. When I was in college I could probably do 5 or 6. Last night I did 17. By June 1st I hope to double that.