David - Thanks for the info! I would hope 40+ would be the exception to the rule, so I'll look into both of those. Ideally, I want a pack that can be used relatively lightweight while still having the weight capacity if longer trips are necessary.
John - I have indeed considered an external frame. I used one for limited backpacking (under 10 miles a go) for years. I sort of like them, but I guessed that a well-rounded survey of what's available was germane before making decisions. Also, I have done some backpacking, just not anything longer like I want to attempt now. Next year I'm going to attempt the Ouachita Trail and see if the interest sticks before planning something more substantial.
I love to walk. I usually walk to work, and it's not uncommon for me to put in 8-10 miles a day just in commute. In the evenings I'll go out for another walk for a mile or two, just to clear my thoughts. I'm not sure why backpacking as a hobby never occurred to me. An ideal vacation for me is one where I can walk, a lot. I find it therapeutic. But at the same time, if I'm going to be walking a lot, I might as well go somewhere in the process.
But your concerns are fair, and I'm slightly concerned myself. I can and have carried a 20lb or so backpack with me when I commute to work, but I've never carried anything over that in commute. The biggest concern to me is how well I acclimate to the extra weight. I would attempt to rent some gear at REI, but Oklahoma is unfortunately devoid of them.
Kevin - That might be useful! There's not a large amount of camping stores around here. I did go to Backwoods in Norman and look at some Osprey packs. They were comfy, I admit, but I'm not really the type of person who would make a decision based on one brief experience.
Also, are there any longer trails in the greater area (besides the Ouachita Trail)? Everything I've found is generally under 10 miles. I think I'd almost have to find something in the 20-30 mile range to make a decent backpacking trip. Like I said before, I really enjoy the walking more than the camping. Camping is fun, no doubt, but I have to imagine that if I only covered 8 miles in a day and made camp, I'd get relatively bored if I wasn't with a group of people.
I appreciate everyone's feedback! It's been most useful.