I spent nearly 5 years trying to determine the neurological cause of this. It seemed so clearly neurological at the time. Gradually, after several years, I realized it was a muscular imbalance caused by deskwork, horrendous posture and a lazy running form.
Although I have now identified the form and posture I was missing, and thus identified the root of my leg atrophy, it is too late. I lost my late 30s in a running sence, going from a 2.30s marathoner to someone who might hold that pace for a km on a track. Also, the imbalance is now, in a sense, engrained: even after improvement, it persists, with one leg and all associated tendons far more active and stronger than the other.
The strong leg would sort of craw and draw me forward - this was also the leg that would give out,not allowing me to controll it after 10-15km. Over time, it became more pronounced and early in my runs and, eventually, walking.