You've received good information already, you'll find FlyerTalkers like to help and have lots of opinions. I urge you to look carefully at flying in Eva Air, based in Taiwan. This airline has a perfect safety record and currently flies 747s to Asia (at least the North America to Taiwan sectors are 747s).

I highly recommend the Evergreen Deluxe product, a deluxe economy class that costs about $200 more than regular economy. Eva flies to Asia from New York, Seattle, San Francisco, and L.A. Their 747s come in several configurations (arrangements of the seats), my favorite is the aircraft with the Evergreen Deluxe located in the nose of the plane. There are other versions with First Class in the nose, OK, but ED in the nose is great for the ED traveler. Here's why: right at the nose, in the front of the cabin they have a huge closet for people to stow their carryons, this closet is so large I can step inside it and walk around. The first three rows of ED have two seats on each side of an extremely wide aisle (further back there are also seats in the center of the plane). For the mildly claustrophobic (like me) or a person with back problems (like you), this seating arrangement is as good as it gets on any aircraft.

The seat pitch (distance from the back of one seat to the back of the seat in front of it and the determining factor in how much legroom you have) is 38 inches for Eva's ED class. Singapore Air has a 31 inch pitch in their regular Economy section, excruciating if you ask me, and 37 inches on the non-stops in their Deluxe Economy. Thai International is touting a 42 inch pitch in their Deluxe Economy new non-stop service JFK-BKK. Each airline has their own terminology for their economy deluxe class, I'm using it as a generic term. Just to get really confusing, Eva has the ED seats on some aircraft, not all. Check carefully if you decide to fly ED on Eva. All the aircraft from USA to Taipei have ED, not all have ED from Taipei to Bangkok. Eva has no onboard "lounge," but I take a walk around the cabin at least once an hour (while awake) and they always have drinks and snacks set out on a galley counter, plus room to exercise a little by the exit doors.

If you're going to be flying back and forth from Asia you need to carefully determine which airline you want to use based on their frequent flyer program. Be sure you're signed up before your first trip and always make sure your FF account number is in your reservation. I apply my Eva miles to my Continental One Pass account, and Eva partners with several other U.S. airlines. Singapore, Thai, and Cathay Pacific have their own programs AND partner with some U.S. carriers. Do consider all this carefully before you purchase your first ticket.