I have problems with my legs. i'm very young and i'm feeling excruciating pain in my legs. i have pain in my knees, shins, heel (anterior and posterior), and my feet (toe joint next to my big toe joint and the tendon that connects my pinky toe on top of t

It's : It's always a challenge trying to give specific answers without the benefit of an examination or the ability to ask you specific questions. That being said, the fact that you can run probably excludes any vascular issue like poor circulation (which is unlikely in a younger person). Knowing you have flat feet, and given the widespread involvement that is limited to only your lower extremities, the first thing i would investigate is a biomechanical cause for your symptoms. If your foot structure is such that it's causing your leg muscles to overwork in an effort to compensate for the structural issues, that most certainly can cause all sorts of muscle fatigue, heel pain, shin splints, low-grade tendonitis, etc. I would recommend seeing a podiatrist and get a biomechanical examination. Orthotics, either a good over-the-counter kind, or a custom molded pair will improve your symptoms dramatically. Also, needless to say, make sure you take enough time to stretch before and after each run. Good luck. Hope this helps.