First route I could think of, good access from central belt via trains:
Ft William – Spean Bridge via the old puggy line
Spean Bridge – Lairig Leacach via the landrover track to the L.L Bothy then quad track climb/singletrack descent to Meanach Bothy
Meanach Bothy – Kinlochleven via a ford of the Abhain Rath then splashy pedal along lochside landy track, take your choice of technical descents into Kinlochleven
Kinlochleven – Fort William via the West Highland Way
If that’s not long enough, detour from Meanach bothy down to Loch Treig (riverside singletrack, tricky) and climb back up via Loch Chiarain then descend (in)famous descent to Kinlochleven.
Lots of great singletrack and wilderness up there, but with a nice stop in Kinlochleven to reduced the amount of food you need to carry.
Edit: I’ve just realized this is almost exactly the reverse of the Tour de Ben Nevis Route, zero points for originality then… 😳