KimC, I've found that performing stamina, like other physical skills, improves with practice.

When I first started busking, I could play about 45 before feeling like I could use a break, and would push myself to complete an hour.

Five months later, I can busk for 1 1/2 hours or 1 3/4 hours with pushing myself.

Having said that, I imagine that gig-performers don't typically put in as many performing hours per week as buskers - so you may not be performing often enough for it to count as an exercise regimen.

Marion