It's a bit unusual to lose toenails at 10K distance (more common at longer distances). Although your shoes feel fine, there may not be quite enough room in the toe box. Toenail damage arises if your toes jam into the front of the shoes as you run. The shoes may be sized a bit too small, or the toe box may be too shallow, especially if you do a fair amount of downhill running. If you have any "additions" inside your shoes, such as thicker insoles, that may also be a factor. As for the right foot...It is common to have one foot slightly larger than the other (by a quarter size or thereabouts). Is your right foot the larger foot?