In reply to Newbie-in-London:
You'll find a path on the best used routes on all the Munros, and most if not all of the Corbetts. Link-ups between hills or hill groups tend to be where the pathless miles come in - that, or when you're devising your own ways up hills, not following standard routes. It's all par for the course.
You can generally tell where the worst bogs are going to be, and take pains to avoid them. But some bog tolerance will often be needed even on well used paths. I would not deliberately plan a route that was bog hopping all day!
Bracken is a pain in summer, but only a feature in the lower glens so you generally get above it quickly.
Deer fences: I try to avoid climbing them since it risks damaging them or me, but sometimes it's unavoidable. Hopefully most will be marked on OS maps but you do run into unexpected newer ones sometimes. Try to scale them at the big corner posts if you can, or run along for a bit in case there's a stile.