As muscles contract, energy is provided to support the action. The body has different sources of energy: glycogen, carbohydrate, fat and protein. The difference between each source is how difficult is to convert it from its state into energy supplied to the muscle.
From a long distance running point of view we focus on carbohydrates and fat.
Scientific studies show that the amount of energy stored as carbohydrates in your body will allow you to run about 2 hours unless you replenish (this is the main driver of the “wall”). This is the reason why if you are training to run a marathon you will have to get used to fueling during your run.
The average amount of carbohydrates required to replenish per hour is between 30 and 60 gr. There are several options in the market with carbohydrates built into gels, sport drinks, etc. All these options have in common that the energy is slowly released, so you should not confused this products with the fast delivery of energy coming from a candy or chocolate cookies (sugar).
Finally, the amount of carbohydrates that your body can adsorb is limited.
You should always take your gel with WATER. You should never take gels with energy drinks as the carbohydrates they have may create GI issues.
Note: What is the deal with Fat?
During the last couple of years there has been a shift to train your body to “run of fat”. The energy available as fat in your body is larger (multiple times) than the one stored as glycogen so running on fat will require you to ingest less energy through your run. This is a good idea due to the GI issues that usually happen when trying to digest a gel at mile 20.
On the other hand, the conversion of fat to energy is slower, so if you want to run your marathon at a fast pace, your body will favor carbohydrates. In any case, training to run on fat (running slow) is a good idea as you can consume less carbs and lose weight if that is your objective, but reaching your best performance (on a marathon) running on fat is highly unlikely