Think of a rack and panniers as investments rather than expenses. And good investments pay for themselves many times over. There is no reason for cycle touring to be uncomfortable or painful. Biking is supposed to be fun, not a masochistic exercise!

Buy used panniers if you need to. That's what I did. I bought my current set of panniers on ebay. Or if you absolutely cannot afford panniers, at least spring for a rear rack and strap your backpack to it.

I sometimes wear a backpack when I go grocery shopping by bike, and fill it after my panniers are fully loaded. Ten minutes hauling 10 or 15 lbs is enough for me. I would not want to do it all day.