Can't tell you for sure what your speed should be, but there are some things to think about:
1. You are probably less than 60% fit, and that might be a hard pill to swallow. I know because I had to admit that some time ago. When I got back on a bike earlier this year after a 8-10 year hiatus, I found myself donked at the three mile mark on a flat trail with no wind and riding a more road oriented bike. Did not go on because I thought I would not be able to make it back. I'm a little older than you, but only about 5 pounds overweight and have always been very active. I found I was not near as fit as I thought.
2. Since I started, I've been riding 3 to 4 times a week, with usually 2 10-15 mile rides after work, and 1 or 2 20 mile rides on weekends. I also push myself when riding to the point where I am always sweaty and tired when finished. My "workout" rides are not leisurely. I've found that my energy levels and endurance have increased dramatically in a short time. This past Saturday, I rode 20 miles at an average pace of about 14.5 mph, and when done felt like I could have done 20 more. Time got in the way though. If you keep at it, I would expect that you would improve your fitness, speed and enjoyment of the bike.
3. The mountain type bike will not be near as fast as a road oriented bike due to the higher rolling resistance of the knobby tires and other things. You would likely have less rolling resistance and greater speed if you change the tires to a smooth tread tire/tyre. Would pick up more speed if you went with a road bike or a road oriented hybrid. The reality is that today's road bikes are incredibly efficient at covering distance quickly. Mountain bikes are good for going up and down hills over rocks and tree branches, but at a slower speed (unless you are young and gnarly).
4. There are times when I'm huffing along trying to go as fast as I can, when a 25 or 30 year old passes me like I'm standing still. They are riding a mountain bike and barely putting in an effort. It can be pretty demoralizing until you admit that you are 60 (or older) and no longer are your 25 year old self, or see that they are on an e-bike.
So, I agree - ride more and don't quit. The benefits are worth it.