You are overthinking this, just go out to the track and do it. You will probably have to first take a certification class, since you haven't ridden track for awhile. After that, just start out as a C and move up as appropriate based on your results. There is no such thing as the right track physique, unless you're talking about specializing in some discipline such as match sprints. Also, you're never too old to compete. I'm approaching my 65th birthday and still compete. It all boils down to your motivation to work hard and hurt a bit.