Severe leg pain during marathon training

I have been training regularly for Sprint and Olympic distance triathlons for 2 years now. However, I started training for a marathon in November, and have my race this weekend.

Admittedly, I increased distance too rapidly...8 miles to 20+ in 3 weeks. Okay, I know that was not smart, but I have backed off and done everything I can (ice, ibuprofen, treadmill, etc), but still can't get rid of this leg pain. My last run went well for 10 miles, then excruciating pain in my knees, ankles, arches, and shins. Could things really get this bad from overtraining alone? I was in very good shape before increasing distance, I'm really wondering what is going on.

I did change shoes around the same time I increased distance, but from one nuetral shoe to another (Pearl Izumi Shine (i think) to Asics Nimbus 9). I am a neutral runner, and didn't expect much difference on my legs from 2 different neutral shoes.

Any ideas what is going on? Last minute advice before the race on Sunday? My goal is sub 4 hours. Without leg pain after 10-12 miles, I will comfortably meet this goal, but with the leg pain I've been experiencing, it could result in 4: 30. I'll crawl before DNF