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Some studies have shown that honey is just as effective as gel in providing energy during endurance events, but since it's not well packaged for carrying on the run, the preference is energy gel. During long training runs, which I define as more than 13 miles, I usually carry 1-2 gel packets with me, but I know some runners who go through 3-4 gel packs on a 16-20 mile run. Take more with you than you think you'll need. You can save the unopened packs for the next run.
When I ran my first marathon, energy bars and gels were in their infancy, and all I consumed during my first marathon was water every 5 miles or so. But research has shown, and my experience agrees, that eating something during the marathon can help stave off muscle cramps and give you a boost of energy. Of course, if you haven't trained well, don't expect 10 packs of gel to save you from fatigue, but strategically slurping one or two, especially in the first half of the marathon, before you think you need them, has helped me in many marathons.