This surely has an analog in life: concentrate your efforts at your weaknesses. This makes intuitive sense: when you pick up something new, the learning curve is steep (i.e., you progress quickly), and then it becomes more gradual with time. That gradual part is the dreaded "plateau."
I have a theory for planning a workout program. I don't implement it because I pay a guy to help me structure a training schedule, but I think this idea might work.
There are really about 5 or so workouts that we do over and over during the year. Strength training (weights or calisthenics), speed work, whether on the track or fartlek-style runs, distance work, tempo runs, and cross-training of some sort. Of course, exactly what each of those are changes as we get fit, or take time off, or whatever, but generally everything fits into one of those categories.
What if every three months you sat down and listed from 1-5 the workout you really wanted to do? Right now it might be (for me):
1. Distance work (how lovely. Just me, my iPod, and a quiet road.)
2. Cross training (a little yoga, a little cycling.)
3. Lifting weights (not really fun, but it's pleasantly narcissistic.)
4. Fartlek or interval runs (at least the volume is low!)
5. Tempo runs (ick. Sounds really hard.)
Then you structure your workout to emphasize numbers 4 and 5, while obviously not completely ignoring the others. At the end of three months, reassess. My guess is that the list will look different. That means you improved.