Great North Run

I used to be a soldier so was always fit, until I developed Osteoarthritis in both knees and suffered a back injury while parachuting. When i left the Army, i tried to keep a reasonable level of fitness but did more gym work with weights and general fitness. I didnt really do much running. A friend asked me to do the New York Marathon in November, so i took on the challenge before i am 40, and I want to do the GNR as a mini challenge before that. I'm not too bothered about a time, I just want to finish without falling apart.