Scotland's greatest challenge route, the Caledonian Way runs the length of the Great Glen Fault, along Loch Linnhe and Loch Ness.
It starts at remote Campbeltown on the Kintyre peninsula: summer ferries run from Ardrossan, which has trains from Glasgow. Quiet roads take it towards Oban, after which a multi-year programme has seen miles of new cycle path built to avoid the busy A828. Crossing to the other side of Loch Linnhe at Corran, and back again at Camusnagaul, means you can avoid the nasty A82 - but check the ferry times carefully.
Paths beside the Caledonian Canal, and the Military Road high above Loch Ness, take you the rest of the way to Inverness. Holiday traffic can be a pain in high summer, but that aside, this is a superb route and not too difficult.