The more you ride, the faster you get, generally speaking.

Mountain bikes are not primarily made for speed, for what it's worth. You'll find the fastest speed in road bikes (actually time trial bikes, but these are not made for general riding).

It's already been stated plenty, but less than 2 months and you are going 20 miles? Absolutely wild. You do you bro, you've certainly got yourself figured out better than we do. Mad respect.

I've been thinking about your post as I was biking to work this morning. I see others have talked to you about tires in the meantime but for on road riding I can really recommend pretty slick tires even on a mountain bike they will make a huge difference in resistance, especially if you inflate them as hard as they can take(look on the side of te tire for something that says inflate to xx psi). Hard smooth tires run nearly as good as racing tubes.

Another thing I was thinking about is saddle height. Most new cyclists put their saddle way too low, which is understandable as you want to be able to put your foot on the ground when you lose balance. However that is not the best thing for transferring muscle power to your pedals. I found that for good force you want to have your saddle so high that when you sit on it and you put a foot on the pedal at it's lowest point, with your foot parallel to the ground, your leg is fully stretched.

This does mean that to put a foot on the ground you have to keep your other foot on the pedal and slide forward off the saddle. The posture of your leg for pedalling will be way better getting more power to the bike and reducing stress on the knees.

Make sure your foot rests on the pedal with the front part, like you can see bike racers do. That allows for more muscles to work on pushing than when the pedal is under the middle of the foot.

Beware that raising the saddle does change the front-back balance a bit as your weight sits higher.

I read you bike down stairs, if it is only one or two steps that is not going to change much but if it's a full flight of stairs that might be a problem.

Frankly I'm 52 years old and have been riding bikes since I was 3 and I have not ridden off more than a couple of steps at a time for the last four decades, it's just not something I wish to inflict on my bike, nor on myself. Hopping down half a meter while at speed is no problem, but really riding of flights of stairs? You are a braver man than me.

Besides purpose built bikes for speed and distance, swapping out tires for a different tread can make a large difference. If you aren't doingactualy technical trails and just like road or flat gravel you can get a tire that has some nubs on the outer sides but a smooth patch down the center for way less resistance when riding. When I switch to my winter tires the bikes is so much slower

hardtail XC bike

This might be part of your answer. Modern mountain bikes aren't nearly as good on the road as vintage ones were.

However, don't replace your bike before you check your technique. Did you get a professional fit when you got your bike? If not, did you talk with anyone experienced about setting up a bike for a comfortable riding position? Bikes do NOT come out of the box or out of the aisle set up well for most people, and it's not always intuitive what the best position for you might be without experience.

Average speed is going to be highest when you can maintain pace. Straight, flat paths with few stops. Are you able to find a stretch of, say, 2 miles that has few to no deviations or stops? Including curves - if you have to slow down and then accelerate again, that's inefficient and will tire you out.

How's your tire pressure? Rolling resistance is greatly affected by tire pressure.

Gears? Are you able to pedal at your most comfortable speed of pedal rotation at a maintainable pace?

I'm by no means a pro - I'm also old and overweight, but 10 mph average over a long ride comes very easily when I compensate for those factors.

Gearing, tires, and geometry make all the difference in the world.

My Transition Sentinel is only geared for mountain biking. It's a terrible city bike. Tons of shock, high torque gears for steep hills, cannot go very fast. But it's insane when you need to climb or descend mountains. It has knobby, 2.4in tires.

My city bike is an ebike, and even though it's a single speed, it's pretty comfortable going between 10-30mph on that gear alone. The battery allows me to haul lots of groceries or baggage (and climb steep hills), and it's tires are wide enough to not get stuck in tram rails or gaps in the concrete road. I have knobby tires to avoid popping tires, but smoother, thinner tires will be more efficient.

Edit: if you have a shock, try locking it out if it has lockout.

I'd also recommend checking out city bikes, such as road, gravel, and upright bikes. There's an incredible amount of diversity, and a downhill mountain bike is about as far from a road bike as one can get. One can roll over a rock the size of a watermelon, the other can coast for meters off of a pedal stroke. Ebikes also are phenomenal as car replacements (or even just as car offsets), but generally cost $1,500+ with tariffs.

It's awesome that you are cycling! If you learned only 1.5 months ago and are already doing 20 mile rides you are doing great. Your body probably needs to adapt to the movement a bit, so I wouldn't worry too much about speed just yet.

City riding is slow.

Pump up your tyres.

Go at whatever speed you like.

Don't worry about speed. Speed is a practically worthless metric, especially when you're starting out.

Your focus should be on developing a sustainable cadence: the frequency of your pedal strokes, and the pressure you exert on the pedals. It's a balancing act. For a given output, the faster you pedal, the less pressure you need to put on the pedals. Speed up or slow down from your ideal cadence, and you just wear yourself out without improving your speed.

I looked to maintain about 65-75 strokes per minute, and not pushing so hard that my thighs would start to burn. Some people prefer slower and harder; some prefer faster and lighter.

The purpose of shifting is to maintain consistent stroke rate and pedal pressure. Speed is incidental: if your cadence is good, whatever speed you get will also be good. That might be 7mph; it might be 17mph. It might be barely above a walking speed while you climb a steep hill in low gear, or 40+mph downhill.

Ignore your speed. Stick with your sustainable cadence. Downshift (and let the bike slow down) if the pedals are too hard. Upshift (and let the bike speed up) if they are too soft.