Lots of folk have done the WHW, some in a day. Be prepared for pain along Lomondside, the rest is good cycling.
The Great Glen Cycleway is currently closed in bits. Uncertain how/when it will re-appear.
SUW – I fancy this too, although I think there are one or two sections which might prove problematic with a bike (like the beach bit at Portpatrick). Might need a couple of road-based detours.
Lots of C2C routes – try google – none of them signposted.
None of these would represent the "best" of Scottish wild riding, but would certainly be a good introduction. I recommend you get a copy of the book "Scottish Hill Tracks" by Scotways. And if you can find "Exploring Scottish Hill Tracks" by Ralph Storer, you’ll be well placed.