13 miles is a healthy commute. Don't sell yourself short. The hardest part of commuting is knowing you have to get somewhere at a certain time, do your job/workout/whatever, and come back.

Use any backpack you have for now. When your back gets sweaty, think about a rack and a trunk bag or inexpensive panniers from Nashbar or other discount store.

When I started commuting, it was on a 30-year old bike that hadn't been well maintained. I found I really liked commuting, but didn't like my bike. So I saved until I could afford a new one. It doesn't have to be expensive, and you can get excellent used bikes from Craigslist.

Happy trails.