To the OP: my situation is similar; I can walk okay for short distances, but long distances or standing is very painful. I was encouraged by the people on the Disability Travel forum to use wheelchair assistance on my last trip. It ALWAYS got me to the front of security and customs (in the regular lines - there were no "assistance lines") and through very quickly. The details are in this thread, including my report at post 26 with a description of both the good and the bad aspects.

I echo the advice of oldpenny16 (who gave me encouragement, too) to get a wheelchair and avoid the stress of standing in line. I don't think showing a handicapped parking permit would work. If there's a long line, you'd have to bypass the other passengers to find someone to show it to, while the wheelchair assistant will take you right to the front. Also, someone controlling access to the screening point might not understand why someone who can walk up to the checkpoint can't just stand in the line for a while. (Last year, I probably wouldn't have understood either. )