Honestly I cannot diagnose properly
a) not having having seen your soft but manly foot
b) not being medical personnel
but I have defitinitely had plantar's warts that looked like they have a dark thing in the center. But even a deep splinter can be tough and since you are running a half marathon and have a good podiatrist to boot (pardon the bad pun) I say go to that person and have him/her check it out.
Keep us posted on how it turns out and what you do.
Last edited by canary52; 01-21-2011 at 04:59 PM.