Gaining upper body muscles arent really gonna help you become faster… you might be able to swing your arms faster, but you dont wanna load on too much muscle. Leg muscles are a good idea, but im thinking more of doing squats with one of those bench bars. I personally think if you load extra body weight that you should keep running (2 miles a day), and i think 15-20 pounds is way too much. I dont know how other ppl feel though.
A sprinter running mileage??
You cannot just develop the lower body and not develop the upper body. It just won't work like that. Some coaches (CF for example) believe that the body as a whole is an organism and strength gained in the upper body will aid in strength in the lower body and vice versa.