One of the dubious badges of honor that come with walking long distances is the departed toenail. If you watched the movie "Wild," based on the autobiographical story by Cheryl Strayed, you may the recall the moment, at the beginning of the film, when Reese Witherspoon, playing Strayed, peels off a boot and painfully removes a toenail that was already on its way out.
I've got a toenail like that right now. The skin underneath the toenail is black and dead, but while the tissue might be necrotic, it doesn't appear to be gangrenous (no funky odor, no spread of gangrene).* The entire toenail shifted while I was gripping it with a nail clipper this evening, which indicates to me that the thing is ready to go. I'm not going to force its departure by trying to rip the nail off, mainly because I don't know whether that might end up being unnecessarily bloody and painful. But I'm also not going to do anything to keep the nail in place, especially if the nail thinks it's time to leave.
*Then again, gangrene comes in wet and dry varieties, so it could be a dry form of gangrene. Either way, wet or dry, gangrene is normally indicative of serious infection requiring immediate treatment, and I've been perfectly fine in the month since I ended my long walk. So I'm betting this isn't gangrene at all, but if something horrible does occur, and I lose my toe, you'll be among the first to know.