My observations on commuting:

1) Backpacks are fine unless you're loading it up with things. I keep a pair of shoes at work so it keeps the bag light. In the early spring through early summer I extend my commute by 14 miles (normally it's 28 miles round trip, now it's 42) and have no issue with a bag on my back and I ride a bike with an aggressive riding position.

2) On my extended commutes I have a Lara bar and a banana and I'm good to go, I also make sure to hydrate well especially in the warmer weather.

3) I use my Conti 4000S and haven't had any problems and I commute in an urban setting so I ride over all sorts of different surfaces.