I have two MTBs now. One is a rigid, rigged as a commuter (fenders, rear rack, et cet) and I use it mostly on roads, sometimes on trails (especially MUPs and rail trails). Like the OP, I like the comfort and the toughness. I've ridden two metric centuries (63 miles) on this bike, and one imperial century (100 mi).

I strongly recommend ditching the knobbies unless you plan to do a lot of seriously 'gnarly' off-road riding. I switched to semi-slicks (continental town & country) and I instantly gained 2-3 mph without changing anything else. They're quieter and smoother and work surprisingly well off-road, too. The only time I don't like them is on a steep uphill on a rough trail.

That's what the other MTB is for (with its front suspension and knobbies). Maybe someday soon I'll finally get a road bike....