I'd agree with most of Mr. Cleetus' advice.
Though if you're talking about off-road, on paths of varying quality, I'd say lower distances than that would be reasonable. Maybe more like 40 to 60 miles per day.
Though yes, it does depend on the terrain, how hilly it is, and what the path surface is like, and how much you are carrying. If you have heavily loaded panniers, they will affect the handling of your bike, so might make some terrain trickier.
Plus how long you are actually cycling each day, ie how early can you bothered getting up, having breakfast, packing up your tent etc (for me its usually not very early...), and how much stop along the way.
As for specific routes, I've done most of the Great Glen Way a few times. I think its rather nice, much of it on forest tracks or tow paths etc, so not too hard cycling. Though there are a few narrowish paths, and some steep hills. But it is only about 80 miles long, so only really a couple of days cycling. You could extend it by linking it up with part of the West Highland Way, though I'm not sure how much of that is ridable.
Or I've recently been cycling parts of the Speyside Way, its also nice, similar sort of terrain/difficulty.