I think strength by itself is helpful but overrated. Having grappled with thousands of people on this planet as the result of teaching so many seminars and teaching so many Jiu Jitsu classes at kickboxing schools, karate schools and the like, it has been my observation that the most difficult guys to spar with are those who had the mix of strength and endurance. Let me say this a different way. The guys I sparred with who had strength were difficult to spar with the first 3-5 minutes. After that, their strength would let them down because they did not have endurance. The guys I sparred with who had endurance – and a tinge of strength – were the most difficult to spar with because it felt like they could keep going and going and going and going—Roy Harris
So how do you develop muscular endurance? That’s actually a fairly deep question because the body utilizes three distinct energy systems (phosfagen, glycolytic, and oxidative), each of which allows your body to fuel activities of different intensities and duration, ranging from steady state sub-maximal aerobic, to activities requiring short intense bursts of maximal effort. Each energy system is improved through a different workout methodology.
For example, lifting heavy weights at low reps is a short-term, high intensity activity which relies on ATP (the phosfagen system). If all you do is lift heavy and slow, you’ll certainly develop strength. But your lactate and aerobic systems won’t develop much, so if you’re tasked with sprinting up a hill or grappling at full intensity for several rounds you’ll likely do poorly.
The short (and incomplete) answer to how you develop muscular endurance is to mix up your workouts. Lift heavy some days, do high rep low rest complexes other days, train your aerobic system some days, and push yourself into an anaerobic metabolic state other days. That, and for us Jiu Jitsu players, grapple a lot. Every sport requires specific physical adaptations that can not be trained apart from doing the activity itself. Do you want to be able to grapple longer? Then grapple longer.
There is also an efficiency component that is rarely talked about. I hear white belts all the time say “I need to get into better shape”. Many of them are already, objectively, in pretty great shape. However, they are inefficient at Jiu Jitsu. They utilize way too much muscular power in non-productive ways which zaps their energy quickly. As you gain experience in Jiu Jitsu you become far more efficient. In my experience, athletes with great endurance are often also very efficient athletes. Great technique and excellent timing allows far lower energy expenditure. So get in shape, lift heavy things, and workout. But technical development and lots of mat time is the ultimate answer.