I would stay away from this one. I have it and hate it. The strap intended to keep the bag on the bike does not work well. It is hard to fasten and I found the no matter how tight I got it the bag would come lose going over bumps, pothole, etc. I have to use bungee cords to strap the bad boy down. Some other panniers actually clip on. Ortlieb has some, I think you can get those all over including REI. some front Panniers work on the back and are slightly smaller if you don't have a lot to carry.

I ride in my work clothes for my 8 mile each way commute. Pants and a t-shirt when it's warm with a spare t and my shirt in my bag. Take it slow in, enjoy the morning, set speed records on the way home.