I used to run to work with a smaller Osprey that i LOVED. It had a space for a Camelbak, and was big enough to hold a change of clothes and other necessities. It wasn't big enough for a laptop, though, or even a file folder (though it would surely hold a purse), so I think I'd go a size up than the smallest. I wish I could still do it---we moved further out, and now I don't have a good route. Have fun!

Good back reviews and lots of other advice at TheRunCommuter.com. http://theruncommuter.com/running-backpack-roundup/ I like my big Osprey (24? 26?), but I tend to carry lots of stuff. A wise tip I once read is to buy a little bigger than you think you need, because the unused space won't add much weight, and you'll have it available when you need it.

The good ones are kind of expensive, so consider buying used from ebay. Totally agree with the suggestion to buy a light.

Welcome to the club! You won't regret it.

I run to work with a small backpack (intended as a large hydration pack, but I removed the bladder parts). My main concern was having a pack with hip and sternum straps, and one that was not too wide, to reduce the chance of the sides interfering with my arms. I am a petite woman.
To cut down on the amount of stuff I run around with, I leave a pair of work shoes under my desk. I an fortunate enough to have a locker and shower room at work, so I only bring a change of clothes, my phone, and my wallet (not my entire purse). The pack is just big enough to fit a stack ofabout 1.5 inches of paper in case I need to bring work home.
This time of year, you should also have some sort of light. I modified a head lamp to fit on the sternum strap of the pack.
Good luck!

I'm going to start running to work and I'm looking for recommendations for a backpack for this run commute. I'm looking for something midsize (big enough to fit a medium size purse), nothing huge and nothing tiny. I'm a woman and 5'6". I don't really care how it looks, just want something that will be comfortable for a 3 mile run. Any suggestions? I'm also really curious to see if anyone else out there runs to work and if so, would appreciate any tips.