I have not cycled the south side of Loch Ness
A road cycling paradise. Stratherrick is a plateau accessed by climbing from Abertarff to the south or Strathnairn from the north. It was a bit of an enclave historically and highly sympathetic to the Stuart cause. I attended Catholic mass there last year and found an actual Jacobite in the congregation.
Besides Tulyar's points we also have;
1) The Inverfarigaig corkscrew - an old coffin road now tarmacked as a series of alpine switchbacks. There is an informal MTB track goes straight down the middle for the brave. Astonishing and little know climb up Glen Farigaig.
2) The Pass of Inverfarigaig to Errogie. Possibly my favourite road climb on this island. You could do Dalcrag-Foyers-Inverfarrigaig-Errogie every day for a year and not get bored.
3) The Dores-Achnabat-Croachy climb beloved of Sneckie's drop-bar crew.
4) The Farr-Garbole road. Ninth highest pass through land controlled by evil persons.
5) If you like a bit of hybrid riding (I don't) then the off-road climb over from Loch Ruthven to Loch Duntelchaig by Loch a Choire and return by Dunlichity and Brin is a lovely loop.
The military roads there are not as early as you might think. Loch Ness was so intractable the Fort Augustus-Fort George (Inverness Castle at the time, not the modern one) link was by a rowed galley before the road up and over Stratherrick was built. The stretch blasted into the rock at loch-side level was a late addition to avoid snow and the course of the high road was changed many times before that was done. As a result there are still off-road sections extant including the remains of a Kings House.