I've been a (FAA) CFII for about one year, accumulating approximately 1000 hours of instruction. I think every professional pilot should start out as a flight instructor, because you will actually learn a lot while instructing, and you will have a great time.
But you mention problems with your back, and that is a big concern, as this is one of the most common health problems among instructors (and commercial pilots), in addition to hearing loss. A common term among the instructors is the 'cessna-back' that you get from sitting in a small airplane in a pretty uncomfortable position 5-8 hours every day.