We do as Kate says and have followed the 5min/per month rule. Alfie is exactly the same age as yours so we are probably facing the same issues at the same time
This probably isn't enough but as a general rule, we do 20-30 mins of lead walking and 30+mins free running each day. Also do a fair amount of training and mental stimulation at other times. I tend to go by Alfie, i have days when i set out planning to spend at least an hour on free running but thats always the day that he decides he is too tired and doesn't want to play

Also if i find that he is getting a bit hyper indoors then we go for a walk
As far as joint problems I have always been told to avoid too much jumping and stairs as this can be the worst culprit.
I am also finding that our general commands are perfect when its just me and Alfie, but as soon as we get to puppy class its all out of the window, must be too many other distractions
Cathy