3 weeks to a month eh? Okay, week one, run SLOWLY. so slowly, in fact, that there is really no risk of becoming winded. rediculously slowly. the key is to get DISTANCE under your belt. cover ground. by the end of the week, you should be able to run 5 + miles no problem, be it slowly.
week two, continue with rediculously slow, longer runs, but throw in some "pickups" from 100 to 400 yards, followed by dropping back to your very slow pace. do this as often as you can, without killing yourself of course. runs should be 3-5 miles.
week three, continue with rediculously slow runs (you'll probably be getting faster by now) but on day two, one mile warm-up and do three one-mile repeats. pace yourself, and do them as consistently as possible, at your fastest pace (use good shoes and soft ground if possible - don't get injured!) by the third mile you'll probably be dying, but get it done. then one or two miles rediculously slow cool down.
next day 3 miles, easy.
next day 5 miles, with the middle three miles hard (80 - 90%), the first and last mile super easy.
next day three miles, easy. at the end of the run, find a field and do sprint drills up and down, 5 or ten times.
next day off, light biking, maybe a small hike.
PFT: warm up super slow for a mile, stretch a bit w/ deep breathing, then bust it out.
YOU CAN DO IT!!