I adore my bike. I have a hybrid. i can ride trails or on the road.
I ride about 10 miles in an hour. I burn 1000-1400 calories depending on my ride. I havent been riding as much because I was so sick in feb/mar but I am starting to ride again.
If you plan on riding alot it would be worth your while to just shop around and have a few different people measure you and fit you for a bike so when you go off on your own you can have an idea what will give you the most comfortable ride. I paid over 500 for my bike but you certainly dont have to spend that much to get started. If you like it then later you can get something different.
Good luck in your shopping!!!
here are a few articles
Luna Bicycles - The Skinny on Women's Bike Fit Womens bike fit and saddles Proper Bike Fit Can Prevent Pain and Injury
There are hundreds of articles out there and you can find info on online cycling magazine sites.