I would stick to sports drink and water to get through your long runs and the marathon itself. Be very careful of using energy gels, more water must be consumed to counteract the dehydrating effect of energy gels.
In regards to mileage and long runs. The long run should not be more than 33% of your weekly mileage. For 17 to 20 mile long runs you would be running 50 to 60 miles per week. More conservative plans have the long run at 25% of weekly mileage. To run 17 to 20 mile long runs you must be running 70 to 80 miles per week.
Sorry but that's rubbish, so you're saying that anyone who is running less than 70 to 80 miles a week shouldn't run anything longer than 16 miles for their long run? I better rethink my whole strategy in that case.
I would say a high percentage of runners will never get anywhere near 70-80 miles a week for various reasons. Most of the plans I've seen, even the conservative lower mileage ones will have long runs up to and including 20 miles.