Could running too hard really start to burn muscle?

Sort of: Running long distances or at high training volume puts a large load on the body. During a long run of 2 hours plus, the body begins to run out of glucose, and needs to regenerate it. One way it does this is by breaking down muscle (protein) and regenerating glucose from amino acids. This is why it is difficult to build large amounts of muscle while distance running. Sprinting is different though.