Actually did a 55 mile ride yesterday! Got into biking about 7 years ago and I rode bike with my hybrid, a Trek 7.3FX, and mountain bike with my Giant Talon 2 29er. Both are excellent bikes, and both came from Cycle Works in Lincoln, NE. I'm currently going to college at South Dakota School of Mines and Technology in Rapid City, SD.
Honestly, I would stay away from the Walmarts, Targets, Shopko's or stores like them if you're looking for a good quality bike. Most of the bikes that come from Walmart and such just don't work quite right. I would definitely go to a bike shop in your area and talk to them. I've yet to go into any bike shop where I didn't receive excellent customer service. They'll definitely first ask you what type of riding you want to do, and then how much your looking to spend. They'll more than likely have a few different models and options for you.
Most likely the bike shop will carry the big brand bikes, Treks, Giants, Specialized and there are a few others. Any introductory bike will be between $300-$500. You might get lucky and get a sale on a bike, but those usually happen at the end of the year. But you can't go wrong really with a bike from a local shop and it's going to last, and be comfortable and fun to ride. If nothing is in your price range, some of the bike shops have used bikes that could possibly be a good option also.
Most bike shops I know of offer free tune ups through the first year of owning the bike. This is because the cables like to stretch when they're new. The bike shops can also offer you maps of trails in the region. Some will even have ride nights where they'll organize ride's for anyone wanting to come along. There are lots of good reasons to buy from a local bike shop besides just getting a good quality bike!