I think its a good idea! If you are up for the planning ahead, then I say go for it.

I, too, have run with a pack. In fact, I sometimes trail run carrying a large camelbak--good training for adventure racing. It's not too bad as long as there is a sternum strap and the pack fits well.

We have a friend that is an elite marathon/ultramarathon runner. And he used to run 13 miles into work on occasion. Sometimes he'd commute on his bike, other times he would run. He carried a backpack.

Wahine has some good advice about the long run and weekly runs. I've followed that philosophy myself and have had success.