I agree with the 20 mile advice. Plus, I would advise putting in a lot of mileage per week in the months before your race. (unless it's a triathlon) If you run 40+ miles per week, you should be good to go.
In my training, I do one long run per week...from 12 miles building up to 20 miles (or perhaps more).
I also do one track workout per week for speedwork.
The "wall" seems to move further out with more mileage in the tank.
I have done the Ironman with practically no run training in the 6 months prior to the race (I was injured). I made it, but it is a lot more fun with the proper training. \
Be sure to do your taper before the race. You can start cutting back on mileage about 3 weeks prior to the race. ...then really cut back race week. (I do practically nothing race week - just some short runs...maybe some pick ups)
Good luck to you!