*types cautiously*
…I think you could make a belter of a route finishing in Glenfinnan, if you were up to just a tiny smidgen of hike-a-bike…
Dalwhinnie train station to Ft Augustus via Loch Pattack, Kinlochlaggan and the Corrieyairaick pass.
From Ft Augustus to Glen Affric somehow (this route exists, I’ve just never ridden it) and finish in the Allt Beithe Hostel or Camban bothy.
From the Hostel descend through Bealach an Sgairne (a famous descent) to Glen Shiel, up and over the Ratagan Pass on tarmac, and round the coast to Loch Hourne and Glen Arnisdale.
Through Glen Arnisdale into Gleann Dubh Lochain and follow the good tracks and VERY steep descent into Kinlochhourne.
Tarmac from Kinlochhourne to Loch Quoich, cross the eastern dam and follow the mostly good singletrack into Glen Kingie, and the Kinbreak bothy. From Kinbreak you would probably need to push up the bealach beneath Fraoch Bheinn before a good quad bike descent into Strathan, then another push up Gleann Cuirnean (not all pushing, some singletrack) to the bealach beneath Streap, before a triumphant, fast, descent to Glenfinnan! 😀