Major congrat's on your weight loss and your new healthy life choices! Having been there myself, I know that it can be difficult. However, I will say that you have made an excellent choice with cycling. I have thoroughly enjoyed cycling since jumping in myself about 5 years ago. Not only is the activity itself rewarding, healthy, fun, and very theraputic (nothing melts away the everyday like a nice long ride) but you will find the people in cycling equally rewarding. It has been my experience that, in general, cyclists are some of the warmest, friendliest, and helpful people around. Welcome!!
To your question:
I would say that the last thing you want to do is buy a cushy saddle for your tushy. Against what would seem to be common sense, the smaller the saddle the better. The logic is that you want a seat that accommodates your sit bones, not your behind. Saddles that fit your behind end up putting pressure on the soft fleshy part of your buttocks and make the ride even more uncomfortable and painful. Having been a larger rider myself when I started, I understand where you are. I can tell you from personal experience that after getting my behind used to riding, the pain disappeared. On a few occassions I have been forced to get off the bike for extended periods and the first couple of rides back are always a break in period for my bottom again.
As the previous posts suggests, you may want to talk to your local bike shop and see if they would be willing to exchange the current saddle for a slightly larger one but just don't go overboard. If they will not exchange it at this point, give the one you have a bit longer before seeking out a replacement.
Remember the purpose of the saddle on your 5200 (good choice by the way, the same choice I made) is to support your sit bones - the bony part of your bottom when you are sitting - not to support your entire bottom.
Don't be afraid to get out of the saddle either. If you find discomfort on longer rides, an easy solution is to strecth by standing up!
That's my $.02! I wish you all the best!