I don't see what's so difficult about the situation. If you enjoy spending time together, then by all means remain friends. Just don't let the friendship get in the way of meeting other girls that you may want to date. Maybe spend a little less time with this girl so other girls around you don't think you're 'taken'. You already stated that you can't picture the two of you being together for the long haul, so why worry about it?
Originally Posted by boymonkey
So your best friend right now is a girl. What's the big deal? It's fine to have female friends, some may even say it's useful.