Actually, instead of the tougher rides like the Aueralm, you can always set off from the same car park (is it the sonnenbichl one???) and head towards Bauer in der Au (sp??) and follow signs for the Schwarzentenn alm. The route has about a 300m in height climb that is mostly very gradual with one slightly steep part. Once at the top you reach the alm which does pretty nice food (and beers!!). From the alm there is a nice downhill until you reach a road , turn left along the road and you head back into Rottach Egern at the southern part of Tegernsee, or go straight over the road to find another off road trail which you can follow to Kreuth and then back to Rottach Egern via an off road route. I think total distance is around 20-30k.
It is definitely one of the easier routes around there... it could even be in the Moser guide for that area, but I think if you buy a radweg/wanderweg map it will probably be shown.
Other tougher MTB routes include leaving Bad Weisee and heading up to the kopfkugl, across the mountains to the marienstein (I think??)... and then down the mountain into Gmund at the northern end of the lake... this is probably a medium difficulty. There are loads of others, some already mentioned in other posts...
enjoy!!!
8LGM