Running

i think most people undereat during the marathon moreso the slower they are, there's science available to tell you how much carbs you will be burning as time goes on and ideally you'd be looking to replace it with gels so that it can be absorbed before you run out.

http://www.runnersworld.com/newswire/more-carbs-better-in-marathons-but-practice-first

I do a gel just before the race, then every 3 miles and have never come close to hitting the wall (although london last year went wrong for other reasons unrelated to fuelling).

it's important to be aware that not all gels are created equal either, this is a good comparison of some of the options... I use GU fwiw

http://fellrnr.com/wiki/Comparison_of_Energy_Gels