Ah. The Outer Loop
Blair Atholl to Aviemore is pretty quick. You should make it there for food easily enough.
Might be worth booking food in Aviemore rather than winging it.
Easy road out of Aviemore and almost all good riding to Tomintoul. Dorback Burn should be low and easily crossable. At Burn of Brown you’ve a choice of multiple river crossings or just make the first one and stay on the South bank. If you do the latter you’ll be fighting through bushes so Smidge up.
Hopefully no headwind up Glen Avon as that’s a killer. Loch Builg track should be firm at the moment. Fast ride to the lunch huat at the foot of Culardoch then a steep climb, Good descent but look out for the juntion when you enter the newly clear-felled area. You want to go left up the less obvious track. Again, might be worth booking food in Braemar.
Easy ride to Linn of Dee, past the red house. Geldie ford should be low. Slow up a bit going down Glen Tilt as the consequences of a fall are extreme. Hard push up to Fealar Lodge then it just “undlates” a bit until the descent to the last ford. Good track to Blair.
In general, 29er hardtails wil be ideal.
If you are on Facebook then you’ll find a few recent photos on the Cairngorms Loop facebook page. I’m also about to pop up a link to a recent sub-24 ride report. You might find some useful info in that too.
The longest day
Edit: as @jimmy says, you can take a shorter route from Aviemore to Tomintoul, past Ryvoan Bothy to Forest Lodge then join up with the official route again. Less road and a bit more scenic.