You will get a ton of different theories about this particular thing depending on who you talk to. I am coming from the side that you are dealing with a skill sport. Usually, the skill is what is holding back your absoulte speed. So, in this light, you shouldn't practice riding slow. What this means is that if you are running motos, you should stop when you feel say a 10% reduction in performance otherwise you are practicing your skills at a slower speed, thus in a funny way you are practicing riding slower. To increase your riding specific endurance, you need to bring the difficulty of the riding down so you can increase the duration without practicing your skills at the wrong speed. This could be riding an easier vet or beginner track, skipping the whoop section or doing it WORCS style and adding a trail section. Then, when you riding specific endurance catches up with your skill level, you are all good.
If you don't have the luxury of altering what you ride, then you can approach it by increasing the frequency (number of motos) and decreasing the rest interval. Say you can only go 12 minutes at your race intensity at a specific track. You wait say 20 minutes usually and do another moto of the same length. Instead of that, do your 12, rest only 5 minutes, do a 10. When that isn't a huge stretch, decrease you rest interval until you can say, run 12 minutes, only rest 3 minutes and then run 10 minutes. The next step would be run 15 minutes all in one shot, etc. Then add another 5 minutes after only 5 minutes of rest and so on.
So the last couple of times out, I practiced as you explained above. For example, I went out for 20, rested 10, then went out for 15 before my arms started to give out. I waited 30 minutes then did another 15, rested for 10 mins, then went back out for another 5 before my hand started to cramp up. Seems to be working well, I think I need a couple more rides before I can give a solid evaluation...
I'm not getting winded, which is much better than a few months ago, and my recovery between motos is also much better than previous. It seems arm fatigue is now my primary issue (not arm "pump"). Anything I can do during the week, to improve on that issue? Thanks!