my legs HURT

i don't participate in extremely heavy cardio.. 30-45 minutes maybe 3-4 days a week (trying to get to 5 now that schools over) i am finding that my legs are KILLING me.. why?! i usually run at a 6mph pace.. and add in 10-20 mins of elliptical and/or 10 mins of bike, depending on how far i run. i stretch too! what gives?

Might be you're not hydrating enough. Drink a lot of water and you won't get muscle cramping. Also, try eating a tablespoon of peanut butter 30min after your workout, or some sort of protein. Helps those muscles recuperate.

I agree with ftay.  You probably need to drink more water.

have you considered that you may be overdoing it?  take a few days off to give the muscles a chance to recuperate.  soreness that lasts more than a day or so is actually from small tears in the muscles (not lactic acid, like most people think).  that's normal (and fine) because those tears are part of the mechanism that lets muscle rebuild itself bigger and stronger, but if you keep working out without giving it enough time to heal itself, all you do is continue to break it down.

take a few days off running and see if that helps.  if they keep hurting, though, you should go see your doctor.  severe pain or pain that lasts a very long time could mean something is wrong.