Pain on underside of foot

Hi all, 

I've been scouring the internet for anything that is similar to what I've done but I have Tough Mudder coming up this weekend and am really worried whether I'll make it.

Basically, I went for a 10 miler last week and at mile 9 the sole of my foot, on the outer edge, started getting painful, and got worse till I submitted to walking the rest of the way.

Here's my symptoms since then:

I was able to walk ok for a while, but became painful later on. The next day I could only walk on my heals or toes (tiptoeing was painless) but not the normal rolling walking action. I can now walk ok, but it has just got more painful again after a day of shopping (due to my wifes need for varying shops in all locations!). I've iced the area which did help but there's nowhere thats painful when I "jam" my fingers into my foot. Only wait bearing I suppose. I have no dull pain. No sharp pinpoint pain when walking. Just masses of confusion!

Please Help

Thanks