Thanks Travs, it's good to know that it may be possible for even my toenails to recover from their current condition. I have tended to assume that there is nothing I can do about the inevitable "wear and tear" of running up and down hills, and none of the broken toenails etc that I have had in the past have been particularly painful; but this one is different, so maybe I really ought to take more care of my feet in the future.