you can replace carbs after a run by either usuing a recovery drink like Rego, or simply having some food
at the distances you're doing, you need neither gels or a recovery drink, but I agree as you go longer they are of benefit. but you can also use other fuel sources like jelly babies, wine gums, Harbos etc - you don't "need" gels although are very convenient to swallow and digest
I'm with cougs on usage - let the body use it's own glycogen reserves up to around 15 miles and then supplement with either a liquid fuel or gels (both if going beyond 20 miles as you need fluid for dehydration). and I only use a recovery drink if beyond 15 miles - otherwise, normal diet.
and if you're running out of juice at 15km, then you're either not feeding well the night before (not enough glycogen), or you're going too fast - slow down and get into fat metabolism to develop more endurance.