I've been looking at these forums some as well as people's setups, and I've noticed that it's fairly common for people to bikepack with a backpack. I did a ~65 mile ride yesterday that was mostly road with lots of elevation change, but included maybe 10 miles of singletrack on my cyclocross bike and I experienced borderline debilitating lower back pain from the first 5 miles on. This wasn't really bikepacking (I was riding to a friend's house for a party and taking the fun route), but I'm interested in doing some trips with a more bikepacking style (despite having a perfectly good Long Haul Trucker touring bike and panniers). My backpack seems common on this site, an Osprey Stratos 24, which I like for commuting and groceries because the webbed back keeps me from getting too sweaty, has good compression, and can cinch down for no sway with the waist and sternum straps.

I'm mostly certain the cause of the pain was the change to wearing the pack, as this ride was not very outside my usual ride difficulty, and I've never experienced discomfort like this. My bike fits me great.

Is there anything I can do fitting my pack that could make this work better for me in the future? Unbuckling the waist made it a little better, although mostly I just had to get up and stand every once and a while to alleviate the pain. I'm going to make a framebag for my cross bike and probably get a large saddlebag for when I want to carry stuff (and possibly camp), but I'm wondering if a pack can be a good option. I'm also just genuinely curious how anyone can do serious miles with a pack cycling. I have thousands of miles backpacking with both very heavy (up to 100lb) and light to ultralight packs, so that's not the issue.