For 6 years I commuted year round with a backpack loaded with a change of clothes and lunch, etc. My commute was 13.6 km round trip, and I was just fine with that backpack.

Then I moved and tried to commute to my new job with the same setup ... same backpack, same stuff. But my commute was 70 km round trip. I used the backpack ONCE, and nearly threw it into the ditch. 13 km was fine ... 70 was definitely not.

My suggestion would be to load up a backpack the way you're thinking of travelling, and go ride the distance you're thinking of travelling every day ... see how you go for yourself.