Let the Match Director know what you are up against. You can walk and gun with no problem. If there is an unusual position that you can't get in to, usually you can agree to do something else and take a penalty. Talk to the MD. If it's a good place to shoot, they will accommodate you.
I'm 73 and still having fun shooting. My body isn't in too bad a shape for my age. I tell people it's because, as a youth, I didn't play football, race motorcycles or jump out of airplanes.