Some things to watch out for from someone who has been there:
REST. Make sure you work in time for your muscles to recover in between workouts. This doesn't apply to cardio, however.
Run. Then run more. Then run some more. And for a change, run after that, too. However, watch out for shin splints. Do what's within your abilities, but always push yourself beyond that comfort zone.
Try to vary what you're doing every once in a while, to keep both you and your muscles interested. Be consistent, though. You'll be tempted to take a day off because you don't feel great. You'll feel better if you just go ahead and work out.
Just my $0.02. I'm not a pushup machine or anything, but I went from being a 98-lb. weakling geek type to being in shape in about six months. I still have a long way to go, esp. for my Marine Corps PFT, but I feel great, and I look a lot better (read: not scrawny), too. The effort is definitely worth it.
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Chris Tessone
http://www.polyglut.net/