OK. With that duration, I’d suggest the following as a loop (there are various other tracks that could be added).
Head out of Aviemore to Lynwilg and climb the Burma Road. Take the descent down to the river along to Sluggan Bridge and climb back up the minor road from there. Head to Carrbridge. Tracks through the woods will get you to Boat of Garten where you can pick up the Speyside Way. Follow that to Loch Garten and then Nethy Bridge. Head to Forest Lodge, then all the way round to Ryvoan Bothy and down to Lochan Uaine. Glenmore, round the back of Loch Morlich, to Loch an Eilean (possible detour in/out Glen Eaniach) then pick up NCN7 south to Feshie Bridge. You could do a bit of a loop in this area before returning via Loch Insh and the Speyside Way from Kincraig back to Aviemore.
All of this would be suitable for a loaded “gravel” bike.
If you want help figuring it out, or added bits and bobs depending on your fitness, let me know and I’ll put together a GPX file.