Jebba, good question. I'm sure you can get a more scientific answer from an exercise physiologist, or a comprehensive running source such as Noakes's Lore of Running
, but I'll take a shot at your question:
To preface the topic of fueling for the marathon, it helps to categorize your needs into the following areas:
- Carbohydrate (energy) replacement.
Some products provide each category separately, i.e. water, table salt, and sugar. Other products, such as Gatorade, provide all three categories in one. When determining your fueling needs, I've found it helpful to break it down by category as not all products are equal.
It helps to go into your long training run fully loaded up on all three of the categories. Therefore, it is a common practice to pre-load the day before a long run with a carbohydrate-rich meal and adequate hydration to suspend the carbs as glycogen into your muscles and liver.
Liquid replacement is factored on your sweat rate. The more you sweat, the more liquids you require to avoid dehydration. Electolytes are necessary to help absorb liquids through the stomach and eventually into the bloodstream.
Running more than two hours, you will have to replace carbohydrate if you wish to maintain your long run pace (it is feasible to continue running without fuel, but expect your pace to deteriorate.)
Timing your in-run fuel-replacement. This is where it becomes individual. Though you can run for about 2 hours without fuel, it is wise not to completely deplete before re-fueling. You will have to experiment to see what works for you. In my experience, it helps me to partially deplete my system before replacing carbs (i.e. I wait until about 70-80 minutes into the run until I take the first gel.) After the initial consumption of in-run carbs, I continue to take a gel every 35-45 minutes afterwards and to the end of the run.
Yet gels (carbs) are only one factor of the equation. Consider dehydration as a limiting factor. You should know how many fluid ounces you need per hour on a typical temperature for your location. You can adjust your needs based on the actual weather on race day. Same goes for electrolytes. Warmer temps means the need for additional electrolytes.
I know this was wordy, but I hope it helps. Keep notes in your running log on what worked. Hopefully by marathon day you will have a plan in place to replenish your fluids, electrolytes, and carbs that will keep you running strong to all the way to the finish. Good luck.