After 90 minutes you will need to top up your carbs one way or another. The Lucozade they give you on the course will do fine, but I get rather sick of the taste of it after a while. I use High 5 gels. They're very runny, almost like a drink, so can be a tad messy, but they're really easy to swallow down and come in nice flavours. Apple is my favourite as it's tart rather than sweet. I find the Powerbar ones far too thick and difficult to get out of the packet, same with the horrible tasting Lucozade ones. The worst for me are SIS, which give me rather nasty stomach issues!
There's still plenty time to experiment now. If you have a reaction it won't last more than a few hours.