In reply to tony:
Did this on a mtn bike a couple of years ago.
The river crossing was the hardest I have done , looked for easier points up and downstream before deciding I could just do it safely at the point indicated on the map. Always easier with a bike for support.
Plan B would have been to take the landrover track SSW? , not got scottish maps with me atm, to regain the old A9 and get back to Blair Atholl by a longer route.