Foods with low glycemic index improves running duration

According to this study eating lentils (or anything with low glycemic index) 15 minutes before a run might delay exhaustion, while eating potatoes (high glycemic index) spikes your blood glucose, but does not help with your time.

The exercise used in the study follows the pattern 5 minutes warm-up at 60% VO2Max, 45 minutes at 70% and then to exhaustion at 80%. I think this represents more of a tempo run rather than a long run, but it is still worth the try.