Mine is a large (54) and using their sizing guide is 5’9″ to 6’1″ and suits me fine at 6 feet tall.

If you want to improve your fitness, just use your mountain bike.
I commute a hilly 12 miles each way on my full sus everyday, but I do have the option of taking some proper off road trails most of the way.

Flat-barred Cotic Roadrat with 2×9 gears, hydraulic discs, barends for a nice stretched out option, full-length guards, plus CX tyres for some mild offroading. Does me well for a 12-mile round commute.