Fueling on the run is a very personal item as what works for me or for someone else may not work well for you.
The first place to look is on the back of the fuel source. It will give you their recommendations on how, when, how often and any other things they think you should consider. It will also note what a serving size is. Most will come in nice little one serving packs - but some come in a multiple serving pack.
In general, one is looking to onboard 100-150 Kcal an hour. This is a range and you may be on either side or out of range. Fuel should be taken with water to dilute the solid (yes a gel is a solid to the tummy) and wash it out of your mouth.
A few good resources include PowerBar.com and Gatorade's SSI site. While these guys will place their product's name in place of fuel or sport drink, their information tends to be correct.
Here is a sample from PowerBar.
http://media.powerbar.com/playbooks/290_running_playbook_3_11.pdf
Back to a general rule to follow. Do not worry about eating on the run until you are on a run lasting 90 minutes or longer. Then start by fueling around the 45 minute mark. As above, take a sip of water, then the fuel then the remainder of a cup (4-6 oz). Do NOT take a sport drink at the same time as the solids from both may overwhelm the gut. Then take a fuel every 45 minutes or so during your run. You will want to titrate to a point where these intervals work for you. PowerGel suggests every 20 minutes, but I think they are a little Ironman oriented with that frequency. My observations are that you will want a gel (fuel) between 30 minutes and 75 minutes on the run.
Do not go out and purchase a lot of any one thing thinking it sounds good. What sounds good in the store or tastes good at rest may be just the opposite on the run. So go for several brands, types and flavors until you lock in on 2-3 that work. Note that what works today may not work in 4 months as weather changes and distances increase. That is one reason for trying to keep 2-3 in the rotation. Though, one tends to be able to broaden their selections in the winter versus the heat of the summer. I find that several folks get tired of their fuel choice in a marathon or longer event. So what we tell folks is to put their best flavors in a fanny pack or randomly long the loops of a fuel belt. That way you are not fueling on the same double yummy mocha chocolate the entire day. You simply reach back and pull out a packet and that is what you fuel on during that stop. I like the newer fuel belt better as I can load a couple loops with my 'goto' packet and have that ready for the last 1 or 2 stops of the race.
Now the final point is once you figure out your fuel strategy, you are going to modify it slightly. Look at the race map for water stops. You will want to plug in you pace and time between fueling and apply that to the race's aid stations. This is only a front half of race issue as most races go longer between aid stations in the first half. For example (it's not published yet but the front half of the marathon is the same as the half) you will expect water at miles 1.5, 3.2, 4.1, 6.3, 8. From there I am assuming one in the race track around mile 9 but that could be as late as mile 9.5 just after entering Bear Island Road. They are about one a mile after that.... the exception being a stop back stage AK before the railway and then a long stretch to the point where exiting back stage.
I kind of wnet long but hope this helps
