I hung up my gumshield last April at the age o 37, not bad me thinks. Now in my opinion the best thing to get that stiffness out is either a gentle swim or a visit to a jacuzzi. I certainly felt better afterwards! I played hooker so i reckon i got a bit of front row experience crouch, touch pause engage......
Good luck with the training, i had two gaps of 2 years during my career, (dislocated shoulder which needed an operation and a snapped achilles tendon which needed an operation). Plan to stay involved with the game, i'm on the club committee, a qualifed ref, first aider, and plan to help