We put together a great route as far as Crianlarich which was almost 100% CX-able.
You’ll need to check your maps to confirm some of this but it went roughly as follows: Glasgow to Drymen via WHW, turn north-east and head to Aberfoyle, I seem to remember some nice fireroads here but it may just be old tarmac, check the map. Aberfoyle to Callander on forest tracks to the south of Loch Venachar. Callander to Loch Lubnaig on Sustrans track, divert onto road then cross road to start forest track climb up and into Glen Ample before descending and rejoining Sustrans at Lochearnhead, follow Sustrans to top of Glen Ogle over old viaduct, then either descend to Killin on Sustrans or follow old railway line all the way round to Crianlarich. From Crianlarich you could get all the way to Glencoe village almost entirely off-road, following WHW/old road as far as the top of Glen Coe before crossing over and following the walkers track running parallel to the road which takes you as far as the junction for the Clachaig Inn.
A short hop on the road to Kinlochleven and you could rejoin the WHW via the Mamore Lodge track, this would actually be rideable on a lightweight CX touring set up, unlike the WHW climb! You can then follow the WHW to Lundavra before turning off and enjoying the truly mental tarmac road that plummets into Ft William.
A great way to extend it would be to somehow get to Corrour Station, the track from there all the way down to Fersit is great, and you can piece together an almost continuous farm track/forest track line all the way back to Ft William, descending the puggy line eventually into the aluminium smelting plant!
If you really wanted you could even tag on the Corrieyairaick pass to Ft Augustus before riding south on the great glen way, that would be an excellent wee extension!