Maybe the inner loop of the Cairngorms Loop http://www.cairngormsloop.net/map.html which is basically Aviemore to Braemar and back. I wouldn’t say it’s all singletrack but it mostly is. A lot of the rest of the route is estate tracks though the initial section north from the A9 through the Gaick is also singletrack.
Of course two days’ hammering it would get you a long distance, like 400Km if you were on the HT550, so what constitutes a day’s riding is quite variable.
I’ve been looking at a loop in the Fannaichs and Fisherfield, some estate tracks, some single track and some hike-a-bike, oh, and that river crossing. The only problem is that to close the loop there’s a nineteen mile road section 🙄