Are you willing to give up comfort for speed? If so, cyclocross bikes are fine. They are basically road bikes that are more ruggedly built: stronger (heavier) frame, better brakes, and wider wheels and tires.
I bought mine thinking it would be a great all-around bike. There's no doubt over whether the BIKE can handle anything; it can. (just like the mountain bike could) It is quite a lot faster than a mountain bike too. But drop bars and a hunched-over riding position is the compromise. The tires are hard too, mine run at 100 psi. That's for lower rolling resistance and higher speed.
As an Athena, you likely wouldn't even be able to ride one. I'm male, 5'8", and 180 lbs. Just a small belly and love handles on me. Even so, my belly is a bit in the way when I bend over to get the lower end of the drop bars. Without changing them out, they can't go higher, and I have to keep the seat high enough to get full leg extension.
So based on what you're saying, I'd recommend one of those skinny-tired hybrids. They have narrower, higher pressure tires for less rolling resistance, but still have a reasonable riding position. Also, realize that only better fitness is really going to make you faster.
Being that heavy, you can't realistically expect to keep up with a group of thinner riders. It cycling, it is all about the power:weight ratio, and to a much lesser extent, aerodynamics and friction.
Use a cyclocross bike as a reward for reaching a goal. Maybe when you get down to 180 lbs., plan to buy yourself a faster bike.
Here's an idea to get started cycling: Commute on the bike, if it is possible. How far from work do you live? Start participating in the Commuting forum too. Lots of us there say the rides to and from work are the highlight of our days.