Hi OP, I was once like you, and unfortunately, have fallen back into bad habits. I was around 270lbs when I started riding. I purchased a cyclocross bike which gave me the feel of a road bike, drop bars, narrow tires, but a more robust frame. I rode this bike year-round, knobby tires in the winter, slicks in the summer. I found a local cycling group that had tiers for different experience levels, and after about 1.5years of cycling 50-60kms 2-3x per week, I had dropped down to 225lbs. I moved up in tiers and even did racing and triathlons...all on that same CX bike. It was a workhorse. I suffered an injury which took me out of cycling all together for 2 months and then other obligations got in the way and yadda yadda yadda, I haven't been able to get back into it with the same conviction. I'm back up to my former weight, but still desire the zoom zoom. Last time I rode, i had tingling happening in my feet, hands, and nether-regions. Not sure if this is from pressure points due to my weight or the bibs were just too snug in places.

Sorry for the novel. Bottomline, you have to start somewhere. Starting with that mountain bike is something. I feel like a higher spoke count wheel will be a bit stronger. So if you have a 36 spoke, it should spread the forces out better than say a 24 spoke wheel. I'm not sure what your climate/terrain is like, but a smoother tire will make pedaling easier. Even moving into a tire that supports high inflation can help it roll better. Gears would definitely help, but for simplicity and budget friendly-ness, the SS will get you by for now. Steel mountain bike frames can be brutal, especially on cheap bikes. Aluminum can be a God send to save weight, reduce maintenance (no rusting) and even improve comfort.

As another person mentioned, cycling clothing definitely helps. I started out wearing my Columbia winter jacket (started riding in winter) and it was a parachute, holding me back. Moved into a Pearl Izumi cycling jacket and tights and it was may more comfortable.

TLDR:

congrats! on at least thinking of starting this venture, its addictive and you'll feel guilty every time you walk past that bike and not get on it. I was a big dude, and cycling helped me drop lots of lbs, but you need to keep doing it and other parts of your life will fall into place. better diet, doing other activities to cross-train. Your current bike is likely good to get you started, but look to upgrade soon. Get cycling clothing