My suggestion is to get a good brand bike from your Local Bike Shop (LBS). It may cost a little more initially but those who have ridden bikes from Wal-Mart, REI or Dicks know they are a huge disappointment. If you get a better bike in the beginning your whole experience and introduction to cycling is going to be so much more fun! You won't have to deal with a lot of mechanical problems on a consistent basis as you will with an REI bike. Giant is a really solid place for you to start, they make the Avail3 which is a wonderful starter bike. If you are looking at the manufacturer prices online don't panic they are almost always higher than what the shops charges. Also the Trek 1.0 is very similar and just as good of a bike as Giant and in about the same price range. Even Cannondale has come out with a more economic version these days called the Synapse. All of these will generally have an aluminum frame and a carbon fork which is pretty standard and useful on an entry level road bike. Another big benefit of going to a LBS is they will fit you (if they don't offer to do this find another shop) and they also typically do a free tune up with purchase of a new bike. This is pretty much essential because bike cables stretch (especially new ones) and this way after you ride it a while and it starts to not shift or brake as easily you can bring it back for them to readjust it. Whether you send $800 or $8000 dollars on a bike they all need some degree of maintenance just so you know. If you take care of it you bike will love you back . I hope that maybe gives you a place to start. Like others have said go out and try some different models out and see what deals the bike shops can give you.

It's really nice to have a cross training sport that is a little easier on the joints. I love running too but sometimes my body just isn't having it and cycling is the way to go. I hope you enjoy it too. BTW there are generally groups of cyclists that get together and ride for fun/competition/beer whatever. This can be loads of fun