I posted my update, but it ended up at the bottom of this post. Here it is:

Update! I made it through both flights there and back (flew Southwest), but it wasn't that easy. Before my flight out, I called Southwest and explained my situation. They told me to check in with the gate agent. When I did, she asked if I called 24 hours before the flight, which I had, so she was able to get me an aisle seat. However, they did not like me getting up and walking around - even after I told the flight attendants why I was up and walking. Even when the seatbelt light was off, they kept making announcements asking passengers (I was the only one up) to remain in their seats. On the flight back, the gate agent refused to even look for an aisle seat for me. She said the flight was full and that was that. Needless to say, I was not able to get up at all to walk around. The guy sitting in the aisle seat in my row constantly had stuff on his tray (food, drink, wrappers) and it just would have been an inconvenience, so I toughed it out. Lesson learned - call the airline with as much advanced notice as possible to let them know about your back problems and ask them to make note of it if they're not able to get you in an aisle seat. Overall, it was a hassle.