Another bike is always the answer. At least around here (on BF, not in my house, lol)
S
Speed and mileage vary greatly from day to day and ride to ride. Over the last year I've ridden two centuries, one was actually 124 miles, a bunch of 3-mile roundtrips to the YMCA, dozens of 10-mile commutes to work (and back) and all kinds of distances in between.

On the long rides, I seem to average out at about 14.5, but hills, traffic and navigation are all factors. In a single day, I've sat in a 18mph pace line for almost 20 miles, struggled to stay in the double-digits on a never-ending string of climbs, and a 3-mile, 28mph 'drag race' with another rider for the last 3 miles of an 80-mile day. (It was a slight down grade on to a long flat in to town, and neither of us wanted to be the guy who sat up first)

My commute goes down in the low 15mph range, regardless of which bike i'm riding. My old XC bike is the go-to for that one, as it has the rack and the big light kit installed. Peak speeds are higher on the road bikes, but the avg speed and total time are almost the same, so that tells me that the MTB doesn't need to slow down as much.

Around here, 'MTB' covers anything with straight bars and 2" wide tires, from full-suspension 'Dirt Jumpers' to high-and-tight XC racers, to 30+lb. 'comfort' bikes. Depending on what you're riding now, some MTB's can make good, versatile middle-distance (40-60 mile) mounts with only minor changes.