Back in the day when MTBs were less freeride and more road-like, I used to run a 24-36-48 chainring setup with a old-school 5-bolt crank and regular MTB derailleur. This gave a top gear of 48-12 or 48-11, which was plenty even for slick tire rides. Shifting was a bit slow up to the big ring, and the chain needed to be long enough that it slapped the frame a bit more, but otherwise problems were few.

If you can find a 46 or 48 tooth chainring for the bolt pattern you have now, that would probably be the easiest way to get the on-road speed that you crave. You'll probably need to mess with the front derailleur to get it to work.