Most of us UK'ers who 'Greenlane' (Use legal to use old tracks / unpaved roads) to get our off-road kicks) HAVE to dual-sport effectively, as we need to use the tarmac roads to join up these Green-lanes.
These Greenlanes are pretty sparse in my part of the UK, but typically a Sunday ride for us is around 100 miles. Half of that being Tarmac.
I've got mine geared on 14/48. Any bikes cruising speed depends on how mechanically sympathetic you are to some degree. I find mine cruises comfortably at 45mph.....50mph if we are getting a lick-on and need to be somewhere in a 'hurry', and i have seen 75-80mph ping up on the speedo when i gave it a burst down a hill once.
On tarmac i try to be a little smoother in general, but especially with the gear-changes to give the gearbox an easier time. Particulaly on downshifting.