You're spinning a 42/11 combo "comfortably" on the flats? that should put you around 26mph @ a cadence of 90...wow..

If you like the upright position of your MTB, I'd explore the "fast hybrids" on the market...typically those with tires 700x32c or narrower, some will use more road oriented components, some more MTB oriented, but either way a 700 tire plus a bigger ring should get you where you need to be...

Your 42/11 with a 26x1.9" tire give you a top gear of 98.3 gear inches

moving to a hybrid with a 48/11 and a 700x28 tire gives 116.8 gear inches (or 31mph @ 90) (Jamis Coda)
moving to a road with a 52/12 and a 700x23 tire gives 113.9 gear inches ( or 30mph @ 90) (Jamis Satellite)