I am also a runner with non-length dependent idiopathic SFN - so sounds like we have some things in common. I think that the non-length dependent aspect is what allows running to be possible for me but I know my symptoms are mild in comparison to most on this forum and most of my symptoms that stick around more are in my trunk area and the running actually helps the sensitivity there (maybe just because other nerves are working?). I also read somewhere that exercise helps SFN because of the hierarchy in how nerves function not sure if anyone has seen this - where the physical activity is causing the large fiber nerves to work more which can reduce the overactivity to the small fiber ones. Not sure if this is true but I do know running helps some of my symptoms (although I do think it causes some burning in my feet after the fact).

Hope you find something that works for you so you can continue to run!