No matter you are weak in running. Allah has bestowed so many blessings that we cannot count. Do you know famous Sahabi Ali Murtadha (R.A) had weak legs in his young age. However when he grew up to good age, he was sent by Rasulullah (S.A.W) to the expedition of Khyber, and he came back victorious. So it may happen that in young age one can have any weakness in his body, but by doing exercise and taking care of health he can regain good health. It is better consulting any physician to remedy your problem.