I'm a girl running around 40 miles a week and a P.R. of 5:00 in the mile. I don't just want to break 5 minutes... I want to run low 4:50s.. how can I improve? I have not done any weight lifting but I do core for 15 minutes 3 times a week. Would adding more strength help? How important is nutrition, and how many calories should I be eating? (also, I would like to take my 3200 meter time down to a 10:25)