Mountain Bine as Training Ride

I'm the same way. After seeing the road rash pictures and hearing the got hit by a car stories, I ride the trail system that our city has developed. There's no question that riding an MTB on a non-paved surface is more difficult than a road ride. I've always wondered if there was a mountain bike to road bike conversion factor that I could use to figure out how many equivalent road miles I'm doing. Most beginner group rides go at an average pace of 15 mph, so I thought of multiplying that speed times the time I spend riding to get a "road equivalent." I currently don't use this formula and just log both distance and time.