I did a mostly tarmac back roads with some cycleways / light gravel / old railway routes: Glasgow to Aberdeen via Lochs Lomond and Tay. Train to Glasgow, Balloch, up to Rowardennan and back, Callendar, Killin, Pitlochry, back down to Perth, Dundee and up the East coast to Stonehaven. Chickened out on last bit of A92 but poss a bit quieter now the western bypass A90 is open.
You get some lovely scenery ok not full west coast or Highlands, but easy enough to get b&bs, pubs, restaurants etc. And simple to get to and from by train with the bikes booked on. Can't remember the mileage but was 200+.