[edit – forgot to post this earlier, I spy some similar recommendations that were posted in the meantime]
I've done similar before albeit with different start/end points but my preferred combination of roads (also riding a cx bike) is into the Pennines, north over the wall, up through Kielder, down through Hawick, Innerleithen, Edinburgh and through Fife as mcmoonter suggests (not too sure on alternatives in Fife though). Then either (1) Aberfeldy-ish, Loch Tummel-ish, out onto the A9, sustrans to Ruthven and along the back road, or (2) via Dundee-ish, Kirriemuir, Glenshee, Braemar, Grantown and the back road.
Using either of those as a basis you could stick in some nice offroad bits in the Pennines, a detour through the Cheviots to the coast or stay inland and use some of the Borders drove roads and so on into Edinburgh. After that there's good off road stuff in option 1 between the various lochs in S. Perthshire (i.e. West of A9) then heading north into the Gaick, or via option 2 into the Angus Glens as far as Deeside and maybe picking up some of the estate tracks in the Cairngorms, finishing off with some great woods stuff either way. Though when you get to planning the off road stuff there is so much potential that would take you months to ride if you linked it all up in one go.