At 245 lbs, you aren't a racer so speed should not be your concern. You should be more concerned with basic aerobic conditioning and burning calories. I suggest you concentrate on riding longer, not faster. You should ride at a pace that is comfortable for the time you are riding. On weekends, you should try to do a long ride of several hours or more as your conditioning allows. During the week, ride for about an hour. You can ride a bit harder on shorter rides.

As you put in the miles, your conditioning will improve and your weight should go down. As this happens, your speed will naturally rise. Just keep riding at the same exertion level (average HR) and it'll all work out.

Time in the saddle is what burns calories.