+1

At least try to borrow a big backpack, load it with some substantial weight, and go for an extended ride before investing in one of your own. My back lets me know that it doesn't like the combination of heavy backpack plus a cycling position, although it's fine when hiking. But I know some aren't bothered by the weight on their back so much. I'd rather have the bike carry any heavy loads. And when walking I prefer a backpack with a good hipbelt for carrying substantial loads.