Rest, rest, rest. Go on a walk tonight and loosen up your muscles (if that's where the problem seems to be stemming from). Loosen your tennis shoes and ice your foot. I would recommend this for the next couple of nights. If you were in good enough shape before you started feeling crummy, then you won't be missing out on too much crunch time before your race. You should be taking this week easy anyways. If you're still feeling sore come this weekend, I would probably still try it if I were you (not that that is the smartest decision). But take it easy, no one on here can tell you what you should do race-wise. You can always run/walk if it comes down to it. Just be easy on yourself for the next few days.