Congrats... and please don't think that your size will keep you down! There are plenty of plus-size riders who are strong and fast!

Now... as far as being uncomfortable on the seat. Have you tried different seat adjustments to see if that fixes the issue? I know when my seat was lowered just a hair, the pain I had between my legs just went away completely. And I was having pain after a few miles.

Finally, if that doesn't work, you really need to get some good padded shorts. This should fix the problem. You only have to buy one pair... just keep washing them (without fabric softner and hang to dry). Save what you can... maybe sell something on Ebay? Offer to clean a friends house? Have a garage sale?

Best of luck!