Thanks Mark. I guess some more info is in order...I'm still trying to figure out his limits, but he keeps surprising me. He's a pretty tough littly guy and he handles lots of weekend hikes in and around the southern AT (usually 6-12 mpd--I feel he could do more, but I don't push). He did a very steep day hike a few weeks ago up 2000+ feet and back down with temps in the low teens (F) and high winds with no complaints.
I agree on the Sierras being a great choice--I appreciate your suggestions for logistics. I don't mind (heck, even welcome) some other hikers, but I just don't want to share campsites every night with Yosemite Valley type crowds. Advice on when and where to avoid mosquitos would help too.
Any other areas that might be easy to get to? Seattle area? Is there decent access to the San Juans, Wind River Range, Yellowstone, Glacier, etc.? Prefer mountains for this trip than, say, Grand Canyon.
Again, thank you!