I think a road bike would make a big difference. I started out with a hybird, and just could NOT keep up with our training group (knowing I was about the same level as a couple of people). I switched to a road bike and WOW what a difference. So much more of my power went "forward" rather that up and down. It made a HUUUUUGE difference for my boyfriend who also trains with us. He was constantly at the back of the pack doing 3 times the work with only a 10th of the profit. Once he got a road bike, no one could catch him.
Don't get discouraged. I am also about 50 lbs overweight, but I was just happy as anything to finish. Now, I can train to improve. Keep on peddling!!!