You may want to consider this forum for some encouragement and support. There are many inspiring stories that people there have shared.
If you ride faithfully-- three or four times per week-- you will find that your ability will increase rapidly for a time. In my own case, when I first started to ride, I found just going a few miles was quite a workout. But within about four months, I went from riding around 5 miles at once to riding 20-25 miles at once, at a reasonable speed. In order to do that, I was riding about 3-4 times per week, and gradually increasing the distance and difficulty. Once I was able to ride 20-25 miles at once, I joined a cycling group at my church, and that was very, very good.
Riding with a group helps keep me accountable. When I miss a ride, people ask me why I was absent. It helps me not to be lazy. So I would also encourage you, once you have achieved a little bit of stamina, to look for a group you can ride with. You want the riders in the group to be reasonably close to your ability, or it will not be good.
Final thought: regarding your bicycle, it is probably a good choice. The riding position will be more comfortable for someone who is overweight (I know from experience). Generally I would say offroad riding is harder than on-road or bike trail/MUP riding, so you may want to consider where you ride. I only attempted offroad riding after I was a seasoned road/MUP rider. Of course, everyone is different, so YMMV. However I think you should consider replacing your knobby tires with road tires if you not riding dirt. Knobbies can add a lot of rolling resistance.