The arm pain is likely 50% grip size/50% racket
The game is so different when using a small, heavy, less stable racket like the T3 as compared to the lightweight rocket launchers of the past 20 years. I don't think 'newer' players(regardless of age) can grasp this unless they do what you did....take an oldie out for a run. The game required so much more just to propel the ball the length of the court; now, as my brother used to say..."If you can touch the ball, you should be able to get it back". This certainly was NOT the case with wood or practically any 'conventional'-sized racket. If you don't hit darned near the center of a T3000(for example), the racket will twist, the elbow will suffer and the ball will flutter like a wounded bird. It really is/was a different game with the equipment available prior to the late '70s.