Words of advice for a first time marathoner going in < 100% (Page 2)

Update:  I made it through....I followed all the good advice and I'll concur that the first 20-21 miles were essentially the half way mark....

My knee started bothering me at mile 5, and from 5 to approximately 13-14 it was bothersome, to the point I was thinking it was going to be a loooong day...Then for some unknown reason the pain went away and the range of motion actually improved....From stiff and achey to pretty normal (at least that's what I was thinking it felt like)...

From approx mile 15-21 I was thinking at least physically I'm feeling really good, just keep the pace where it needs to be and I'll be good...

At just over the 21 mile mark my legs just hit a wall....Within what seemed like 100 feet my legs went from "normal" at that point to both quads hurting and feeling heavy....My left leg didn't even seem like it wanted to raise properly...Both hammies were tight and the bottoms of both feet were hurting...

Long story short, from mile 22-25 I ran walked....When I stopped to walk I would stretch my legs for 30 seconds and then walk, then stretch until I got the mental strength to run again....I did that back and forth until the 25 mile mark which I had told myself I would run from there to the finish which I did....

INCREDIBLE experience....If I do another it will be at the end of a 140.6...

Thanks again everyone, I really did benefit greatly from the advice....