I've cycled on the Bodensee Koenigsee radweg, which (though I don't know for sure), I imagine will be similar terrain. It's rough in places, though deep pine forest, with gravel surface for a lot of the way rather than tarmac. I was on a 700c tourer with thin tyres, and though in places had to get off and push, mostly it was manageable. Depending how you choose your route, you might find yourself using the Iser radweg (from Munich to the Bodensee/Koenigsee Radweg). It's a lot easier, seeing as the ground is pretty flat. It can be confusing in places, though, so watch out for the little green bike signs at the junctions to ensure you're still following the correct route. Enjoy, it's a glorious part of the world.