If you're already at 15 and finding them good, you'll be fine, it's not necessarily recommended, but from a mental point of view, i found cracking that 20 mile mark in a training run really useful, but as i say that's not necessarily recommended.
Just looked at my notes for Manchester marathon, my fueling strategy was:
As said that's all Maurten, if you are getting on well with SIS then that's great, stick to what you know, as for carrying them i wore a flip belt, they're about 30 quid, other similar options are available that are cheaper, plus you can get shorts that have enough capacity for gels too, i found the Flip belt pretty good, good idea to practice on your long runs.
Flipbelt link
Another option might be to use what gels they are giving out on the race day, you don't have to carry them (or as many) then, but i don't like that idea as it may mean you miss an aid station or that they run out, then you are buggered!