Short of surgery, you're not likely to get rid of all of the slackness of the skin, but you're lucky: for a younger person, the skin is more likely to rebound. (As an older weight-loser--I've dropped about 1/3 of myself in recent years--I've accepted that my "draperies" are here to stay.)
Insanity, as I understand it, won't do much for the slack skin. A bodybuilding-type lifting program may pick up *some* of the slack, but a) how are you going to put on 20 pounds of muscle in your midsection, and b) if performance on the track is important to you, a lot of bodybuilding muscle will seriously affect your strength/weight ratio.
[Nevertheless I do recommend lifting to you, but the kind that's appropriate for a runner: heavy weight (after you've developed a sound technique with light weights), low reps (3-6 range), and just two or three "work" sets per exercise (you may also need one or two warmup sets for a given exercise).]
Given that that you dropped the weight really fast--but you're young--I'd expect that you'll see the skin tighten up some over the next couple years, no matter what.