Theirs a lot of really great advice you've already received. For 100 miles a drop bar bike will be your savior. Alot of people around here aren't a fan of bikes direct because they have sketchy business practices (like buying old 70s bike brands using them as there own) but there bikes are great deals for the money and their service is great. BUT first time buyers should not buy bikes online unless you already know your CORRECT bike size.

For $500 you can get this bike which has more of a road bike feel as the seat is higher then the handle bars and has a steel frame which absorbs the bumps in the road better then aluminum. http://www.bikesdirect.com/products/mercier/galaxy.htm

Or for $100 more you can get a touring bike from them with a steel frame but has more of a relaxed position with the seat at the same height as the handle bars. http://www.bikesdirect.com/products/windsor/tourist.htm

To sum it up for about 500 your looking at the used market or sale market. With bikes direct you'll have to put your bike together yourself but you can always pay your LBS to build it up for you and check if everything is alright.