OP, you know, I was, maybe sometimes still am, a competitive cyclist. I have owned and own several MTB/ATBs, and I swear, sustained 14 MPH on the fat, low pressure knobby tires they are outfitted with, that is about it. You need a cyclecross/gravel bike with 700c wheels and tires. I can maintain significantly higher speeds on my road bikes and my Surly CC vs my no suspension 26 inch, fat tire MTB or my hard tail Stump Jumper. The right tool for the job.