It's excellent for the train (I used the sleeper from London). Be aware that the waiting room at Aviemore was closed on Sunday, so I had to wait on the station for the train.
Blair Atholl to Aviemore is just under 40 miles and took me 2 and a half days. I pushed the first day, so I did 18 miles, but could have stopped a bit earlier (better camping sites after Loch Tilt and before Bynack Lodge). You could also camp before the Lairig Ghru, which would make it into a true 3 day trek. I didn't know how long it would take and the weather was looking a bit iffy (had a snow shower!) so I pushed through the LG and had a lazy last day. If anyone wants a first trip in the Highlands, this is highly recommended with easy transport, great scenery and easy route finding, with plenty of places to camp. Let me know if need any more info.
My September trip will be a four day circuit from Aviemore to Strath Nethy, Glen Avon, Glen Builg, up the River Gairn, Glen Quioch, Glen Derry, Fords of Avon, east of Bynack More and back to Aviemore (I hope!).
I will also be testing some new kit 
