To have sore back after an hour Greenmate is not quite normal.
either you've got a ****ty harness or bad position or simply has you said the motorcross wasn't the best thing for your back.
I don't want to sound like an old granny but just that I had to go through quite a tough time with back pain and then surgery that I wouldn't wish anyone to have the same... (and I am not that old ..only 30)
THAT IS WHY I would simply advice you to do just few back stretches before going in the water. (and also when you are doing breaks if you feel sore). I know it is sometimes frustrating to be all ready to go and have to spend few minutes doing stretches and to maybe look a bit stupid....but not that stupid when you know that you would be able to sail longer this time and also up to your 70ties.
I used not to do anything before and just go for a beer after , but when you think of it going from nothing sitting in the car to blasting at 20knots in choppy water can't be the best for you body....

so ok I could imagine few people saying what is this crap? I am ok!
Ok but everybody's back is different and you could have the strongest big muscles in your back and still have sore back...see surfer Mark Occhilupo or swimmer Michael Klim
so don't mess up with your back!!
get a good waist harness
do few stretches before going in
rip craaaaaaaaaazy !!!!!!!!!
and have a good long hot shower after
ALL THIS MAKES a HUGE DIFFERENCE
you will sail longer!!! (could still have the cold beer in the hot shower)

here are few ideas:
http://www.soc.org/opcam/07_fw9596/mg07_saltman-b.htmlgood luck