PAIN!!! AAARRRGGGHHHH!!!!!

Friday I ran in hopes of tuning up for the 25k on Sunday. It was around 60 degrees outside. Cold for Houston standards.
I made it out to the turnaround point for the 3.5m and all was well. I make it to the 4.5m turnaround and feel a funny twinge in my left shin. I stretch it out and get going. I make it to the 6.5m turnaround--last stop on the trail--and that twinge is back, except it's worse. I stretch it out and get going again. I make it all the way back to the 3.5m turnaround point and I have to stop and stretch again. Now it's not just a twinge, but an actual "ow" kind of pain.
I make the decision at this point to bypass the 8+mile option and head home (that's the left turn versus go straight decision I talked about last time).
I get home and can't really walk because of the pain. This isn't good. Not good at all.
I don't know if it's mechanical, physical, or simply an issue of conditioning. If it's mechanical, I can probably fix it pretty quick. If it's conditioning, I can fix it pretty quick and I'm simply not ready for a 25k yet. If it's physical, then we have another issue entirely.
I went the cautious route and skipped the 25k since there was still pain on Saturday. If the 25k would have helped my conditioning and gotten me a 4:30 marathon versus a 5:00 marathon, I'm willing to drop the 30 minutes to ensure I'm healthy enough to run the whole thing. I haven't come this far just to get hurt.

Next run: 4.5m (I'm dropping the 3.5m runs, I think) either today (Monday) or Wednesday. Then 6.5 and 8+. Also, a shameless plea for money!!!