How do you build endurance

Are there any excersizes to build endurance for long periods of playing? When I have to play for several sets my fingers start to go numb on my left hand and my bow hand gets really tired. When I stop playing, it all returns to normal
I know that my age is against me. I am 66. I played piano thru my childhood and switched to classical guitar which I played until I was 50 and I picked up my first fiddle. But I have cut to part time at my regular job and gig (fiddle) in a couple of bands to earn extra money. If I have to keep working I want to do something I love and music is IT.