Is 14mph on a MTB with semi slicks an okay speed?

I have a Giant Boulder entry level bike and thats my average on a 6.2mi run/27min. Just started riding about 2 months ago. I am pretty out of shape at 5'6" 210lbs but I have always been very stocky/bigger than most and I ran a 4.8 40yd in HS at 175lbs(my ideal weight). Should I slow down and ride for longer?