Congratulations on you decision to improve your physical condition and your new bike! I've struggled with obesity my entire adult life (currently at 370 lbs) and the only times I dropped a significant amount of weight and kept it off were when I was riding regularly. For me, healthy eating and bicycle riding just go together. If I do one, I also do the other. If I don't do one, I don't do the other. Like others have said, diet is key. Watch the calories and focus on foods that don't stimulate the appetite, which for me, is high proteing. Carbs and fat tend to stimulate my appetite so I moderate them with lot's of protein and I can comfortably eat less.
I'm not familiar with the model of bike you bought but I got my first in-person look at Jamis bicycles yesterday and I was impressed! I've been a Trek guy since 1995 with Specialized being an abstract "second-place" due to the relative scarcity of dealers around here (Trek seems to own this city). The Jamis bikes were gorgeous, looked very well built and spec'd and I was really impressed that most had what I believe were pretty high quality tires. I don't generally expect that from stock bikes in my price range.
Also, ride more days than not each week and your butt pain will subside. If not, buy a new saddle and try again for a few weeks. It's hard to believe when you are hurting but the break-in really doesn't take that long.
Oh, and post pics of your new ride!