Wearing a backpack (or hydration pack) for long distances needs a proper fit (of the bag). Straps tensioned properly and the correct length, and all that. People have used backpacks for as long as 600km audax rides, probably even longer, but the important thing is to get the fit right. I've experienced shoulder and back pain simply because one side was tensioned a little tighter than the other, which took me a quick stop to adjust and fix, but it's better to get everything right from the start than to experience and figure things out during the ride.