Couple of bits of road but the vast majority is gravel. Not my route, when they did it last year the weather was horrible!
Route out to Aberfoyle depends on how you want to get there. You can just hit it straight up the A81, I ride it a lot and it's really not that bad, especially if you're off reasonably early. Otherwise you can go out to Drymen via the West Highland Way, over the pipe track and on the cycle path to Aberfoyle. Significantly more climbing/longer (I think) but a lot less cars. Pretty sure all the WHW is CX-bikeable to Drymen and it's been dry so shouldn't be horribly muddy like it can be. I can plot that route for you if you want it?
Here you go: