The magic formula for going several hours is Hammergel every 30 min, Saltcap on the hour and 5 oz of Perpetuem every 15 minutes. Making that happen is not as easy as you might think. After about 40 miles, you can't trust your own judgment. What was routine in practice gradually becomes a lot trickier. So you have to have systems set up to make it as easy as possible. And practice on every run.
I used a GoLite double bottel waist pack with a GoLite hand strap. I used three 24oz bike bottles (Specialized) one Green, one Pink, one Blue. Different colors are helpful in keeping track of what's in each bottle. Also, green is my favorite color, pink represented my daughter and blue my son. These bottles have a clear stripe on the side so you can easily determine how much fluid is left in each.
Measure and mark the bottle in 5oz increments. Since I'm 6'1" and 183 lbs (on race day) with a significant sweat rate, I was takin in 24 oz per hour. That's a little over 100 cals per hour through the bottle. By using a hand bottle, you can have a visual reminder of your progress against the 5oz every 15min. It's amazing how much you forget to drink if the bottle isn't in your hand.
Taking a Hammergel every 30 min gives you 200 cals through gels for a total of 300/cal per hour. Amazingly this is all you need to go the distance. I used the messy plastic packages. The advantages are you get portion control and a variety of flavors. I would buy 100 at a time from www.zombierunner.com, pull out the caffienated mochas ones and then randomly draw them from my waist pack. A fun game is to not look at the flavor but to try and guess it. I keep the Mocha ones for night runs. I'm not a big fan of the banana or mocha flavors. It would suck to get those back to back on a night run, but then maybe you'll get a Mountain Huckleberry and all will be well. I've since switched to the the 5 serving gel bottles. They are cheaper, cleaner and no trash. One of those bottles and the Perpetuem routine above is a 'tankful' on which you can go 12-15 miles quite comfortably. Even 20M. On one of my 50M training runs I ran out of fuel and was able to go another 2 hours without incident until I could get back to my aid station. Beware those bottles can get heavy. I took three with me to Hawaii to attempt a self-supported ironman, and the weight was significant.
SaltCaps every hour keep the cramps at bay. This rule works well for winter training in Colorado. When I did a 50M race in warm temps in May, my legs cramped just past 25M. I had forgotten to increase my consumption in the heat. I would go to 2-3 per hour and the cramps cleared. I find this also worked in Hawaii when biking long distances. In Leadville, I erred on the side of too much salt in the first 50. My hands had swelled and I even gained a pound or two at the 50M weigh in. I skipped salt altogether for 2-3 hours until I felt better. Then it was cool and nighttime so I went back to the 1 per hour until morning.
I found that running with a Garmin 305 was very helpful. I would use it to track the .9/.1 run/walk breaks. Also, with the clock I'd follow this pattern: top of the hour was SaltCap, hammergel and drink. by :15 it was make sure I was down 5oz on the bottle; by :30 it was hammergel/drink by :45 it was making sure I only had 5 oz left and then top of the hour repeat (grab new bottle from the back). It is fun to practice all of this while moving. It should go without saying that all of these things are done while moving forward. When you set up your aid stations, minimize your time there. I think of it like 'hot potato' I want out of there as fast as possible. That's where failure (stopping) might happen. Get back on the trial. Ideally, you'll have a fresh set of bottles at the aid station so you don't have to mix and pour. This is a great example of something that is easy when you are sober, but not so easy after several hours of running. Come in, dump your trash, grab your gels and bottles and keep going.
Allen Lim, of Skratch Labs, is a professional at all of this. I used his hydration formula at the aid stations in clear Nalgene bottles. This is in addition to the Perpetuem above. Allen talks about your 'clear rate' that is how much fluid can go from the stomach to your body. 5 oz is a good rule of thumb, but there are ways to enhance it with the right mix. If you take in too much, it just comes out the top. :)
I thought dehydration was a big risk for me given my sweat rate, so I would drink as much as I could of Allen's formula at the aid stations to make sure that I wasn't getting behind.
Deep into an event, the perpetuem tastes not good. I'd try using Allen's hydration (which is close to 100 cal); it is much lighter and better tastings. Or do 50% perpetuem solution and make up the cals with more gel.
Whenever I follow this routine, things go well. Through 40M of Leadville, all went well. Then at the TwinLakes outbound station, I grabbed chips, cookie, coke and whatever. I'm not sure why. It looked good and I figured, 'couldn't hurt'. Wrong. Halfway up Hope Pass, I was in big trouble and unable to continue. I eventually walked up to Hopeless, got some soup and then the adventure began. :)