I thought Runner's World explained it best: If you plan to run more than an hour 15 minutes, take in carbs within the first 30 to 60 minutes. Continue fueling with 100 to 250 calories per hour, which is about 2 1/2 sports gels or 16-40 ounces of sports drink.
In an Ironman, I have found it easier to consume solid food such as Shot blocks or energy bars on the bike because I was able to actually chew and swallow as I rode. But on long training runs, I prefer taking in calories as liquid or gels. I bring gels (Honey Stingers) on any run longer than 10 miles and space my intake evenly within the time I train.
Running while eating "real food" just doesn't work for me, so at races, I avoid the cut up oranges or bananas at the aide stations and reach for the gels.
Research also shows that taking a gel 15 minutes before the start of a race or even a swim workout is beneficial.
Hope that helps!