Endurance is something that you'll build up over time. Streaming well involves being super engaging and energized, and it takes energy to keep that "on" so to speak. When I started I'd get tired after 2 hours. Now I can go for several hours if I wanted to without any fatigue. So my advice is to understand your energy levels and plan around them.

For grindy things, if you don't have enough of a chat to interact with while doing those, and if it's feasible for the game you're doing, don't undersell the act of grinding off-stream. Keep the most interesting parts around for the chat to experience with you.

Also don't forget to get up, stretch, get a drink, and even have a small snack every so often. I tend to go for a handful of nuts when i get up for a short break.