Hey BB-
I was able to lose almost 40 pounds by cycling last year! And I still got more than a few to go! So don't worry!
A couple of things I noticed.
My legs felt weak at first. So- and don't laugh- I would cycle around the block- just around the block - sometimes as fast as I could go. Sometimes trying to stand up all the way.
I rode to the coffee shop in the morning, I rode when I came home from work at night.
When I went to friends' to babysit their cat- for a week-I took the bike, parked the car and commuted. Went EVERYWHERE by bike.There was this one mama hill outside the apartment- I couldn't make it at first- but by the time I left- I conquered that sucker! (cue "Rocky" theme) I'd deliberately ride a long distance- knowing I had to ride back, and couldn't wuss out...
It didn't take long, I felt the muscle building almost immediately!
In additon- I dranks LOTS of water...no soft drinks or sugary drinks. Keep hydrated.
And I changed how I ate- I thought I ate nutritious meals before..
but:
I started eating smaller meals 4 -5 times a day.
Lean meat - grilled on a George Foreman grill- lots of salads -
I found I did poorly eating potatoes, better if I ate rice- but best if I ate pasta.
I ate enough to not be hungry. And I still had snackie stuff- If I craved chocolate- I ate it. Maybe not a whole bar- maybe a few squares to kill the craving. Yogurt, soy beverage, fresh fruit.
I started yoga - which REALLY helped with the muscles and flexibilty.
And when I was riding- I didn't feel like I lost weight that day- but curiously the next day- I'd feell- lighter somehow.
I went down 2 sizes in a month...
and don't get suckered into the pounds loss myth- you might be building muscle- and muscle is heavier than fat. so- look at it this way- not- I lost X number of pounds- but my pants are looser, I FEEL better, I can pedal standing up for 5 minutes as oppsed to 2... so look for the REAL progress--
you'll do it!

Good luck!

Kimba

oh yeah- and you could try a ladies seat with an anatomical cut out. You can find Lemond ones on Ebay for 7 bucks - it's worth a try. I love mine- and it is so comfortable . 2 hour rides- 2 and a half- no problem...