advice for 10K swim and lower back pain

I signed up for a 10 K swim in Atlantic City this September and am wondering if any open water swimming veterans out there have any tips for preventing or at least moderating the lower back pain that seems to always be triggered in me by very long distance freestyle events like this. I did the Chesapeake Bay swim in the late 90s, and getting out of the water at the end of the race was like a parody of the Evolution of Man from a squid to Homo Erectus, only in my case, I didn't quite make it to the standing position before collapsing in a heap of lower back spasms. In any event, I suspect I will just have to suffer through the discomfort and hope a week or two of convalescence afterwards will restore me to my self. But again, any advice short of oxycontin shots in the lumbar region will be truly appreciated.