Welcome!
I just got back in to MTBing after being off for 10 years. I spent the past 4 years losing 110lbs. It was quite a struggle. I started riding again 2 weeks ago since despite running 30-40 miles per week, I stopped losing weight and was stuck at 210 for 2 months. I cut the running down to 15mi/week I have rode about 160 miles in the past 2 weeks and dropped 6 more lbs.
The keys to losing fat is diet (very important), lower intensity cardio, and weight training. I think that is why I stopped losing weight since running is higher intensity, it builds up cardio but does not burn fat. I use a heart rate monitor and when I run, I am at 68-82% of my max heart rate. When I ride, it stays around 55-65% and goes as high as 75% on climbs.
Get yourself a heart rate monitor and keep yourself in the 55-65% range and I think you will see results.
Congrats on getting back into riding. I am having a blast getting back out on the trails. I don't know why I waited 10 years to get back in to it.
-David