One thing I neglected to mention in my previous post. If you have back problems it is important that your pack have a good suspension system; one that will transfer most of the pack weight to your hips. Even if you need to choose a heavier pack, the load transfer is worth the added weight. I have used several different UL packs over the years. Even with loads less than 15-18 lb, none of them transfer enough of the weight to my hips to reliably avoid back pain. I have a herniated disc that will flare up if I use any UL pack (I have so far tried) but gives me no trouble when I use my 3 pound GoLite Quest. Pack, shelter and sleeping bag/pad are all critical items for an older hiker and they should be chosen only after a lot of research and, if possible, tryouts. If your pack makes you miserable during the day and you can't get a comfortable, dry, warm nights sleep then backpacking can be just an elaborate type of masochism.
May I walk in beauty.