Gayle -
I just completed a 100 mile ride on my bike in July. I spend a lot of my time on my road bike and I love it. As you get more used to riding and can go faster, you can get your heart rate up and keep it there. The only thing that burns more calories for me than road biking is running - and I can ride a heck of a lot longer than I can run.
First things first - go to the local bike store you trust. They are the ones who will sell you the bike that is right for you. Figure out a budget - for a decent bike expect to pay between $800 to $1500. Prices on bikes did go up last year by about 20%, but it's also end of season and new bikes are out - so you may be able to get a great deal on a 2009 or even new 2008 model.
Then ride as many bikes in the store as you can. Even the ones outside your price range (for comparison - there is a big difference in the way each kind of frame handles on the road - it's nice to know what's out there). What you are looking for more than anything is fit. You want a bike that fits you. Believe me, you'll know the bike that fits you as soon as you are on it. You don't want to feel too stretched out over the handle bars (many brands offer Women Specific Designs -or WSD's in their more popular lines - try them as well if you can). You also want to make sure where you sit over the pedals is comfortable. Saddles can be adjusted, but that range is limited. The actual brand of the bike is not that important. My current stable includes Cannondale, Surly, a couple of Treks and a Klein. I love them all.
My advice to you is to buy as much bike as you can comfortably afford if you think this is going to be a long term thing for you. There is a lot of difference between an $800 bike and a $1500 bike, but a lot less between a $2000 bike and a $5000 one. But an $800 bike is so much better than a $400 one.
Keep in mind all of the fun accessories that go with bike riding:
* Helmet
* Gloves - I highly recommend
* Water bottle cages and water bottles (go thermal - REI carries them - they keep the water cooler longer - and get two cages and two water bottles)
* A new saddle - most stock saddles are painful - expect the spend another $75+ on this - your Local Bike Store should be able to help you measure your sit bones to determine which saddle is best for you
* Possible bike shoes and pedals - you really do get better leverage and full rotation with clipless pedals - but that is something easily added on when you are ready.
* Bike shorts - I personally think a good saddle is far more important than good bike shorts (I can ride my saddle for hours with no padding, but I have been on saddles were no matter how much padding in my shorts, I'm in pain after 10 miles).
The ladies over at Team Estrogen (
http://forums.teamestrogen.com/) have much more wonderful advice. Go to the "New Rider" section and do a search on finding a bike. There are all sorts of riders over there and all sorts of excellent advice. It's a good group.
Have fun!! I now ride with my daughter and I'm so happy to be able to do that.