I had to give up work too I was unable, it wasn't safe, practical or economically viable for me or the employer.
I do like to try and keep up with my Hobbies though that's me striving to maintain myself and ability, mine happens to be Angling, I carry my gear (bit like weight lifting) for a distance where ever i want to get too (stamina) and then there is the technical side of it tying rigs/knots and presentations giving my Mind a work out and at the same time fighting tremor too attain and maintain fine finger movement.
But as Gus points out there are Limits and i know when i have reached mine, I try and work within them, but still giving myself Goals that i still can, that's important too me, to maintain some sense of normality of life, it has become harder, but i have too look at way too go around or do things in different ways innovation which also exercises my mind.
If i over do it I do pay for it and it does take me physically time to recover that is something too keep in mind when finding 'your limits'.
You can apply it too your hobbies and interests. little steps big gains.