Running my last few races I learned that the mind is primary not the physical.
When I bonked after 16 miles at The North Face 50K last weekend there was a physical component but I know that the governor of the whole experience was my own head. There were plenty of times when I could have run when I walked. I walked because it felt better to walk and it hurt to run.
I also notice how when things get tough it can be all too easy to just get down on allow negative self-talk to creep in. Last week I actually got angry at the course for being so ridiculously hilly and muddy! Once the downward mental slide begins it is tough to stop until it runs its course. For me that took about 2 hours and 10 miles.
Mental training is very tough and something we are not programmed to do. We avoid it because it really pushed us past our comfort zone. It mandates that you intentionally do things that are uncomfortable and outside of your normal routine. If you are only doing the type of regular physical training that your are used to doing, then you are not pushing your mental boundary.