For the past week to two weeks I've been having this really terrible burning sensation in my legs. At the beginning of my runs my legs feel stiff and then as the run progresses the burning sensation becomes worse until I have to stop because it feels like they're on fire. Even when I'm just jogging this sensation occurs. I pretty much feel it at an pace. I really have no clue what's going on; I hope someone can help me because it's ruining my runs and training.