Well the first thing to understand is that to have good power-endurance, you first need power!
I'll use sprints to show you how quality must be developed before capacity.
Let's say that you want a 200m sprinter to run an average of 9.5m per second over his 200m sprint. Well he first needs to be able to at least run at that speed to be able to maintain it. If the top speed he can reach is 8.25m/sec he can do all the speed-endurance work in the world, the best he can achieve if he doesn't improve top speed is to maintain an average of 8.25m/sec over 200m.
If you want to be able to maintain a certain level of a physical quality (strength, speed or power) you first need to maximize the development of your peak output in that quality.
Let's say that you need to produce 1500 watts of power for 3 minutes... well if you have a hard time reaching 1400 you wont be able to maintain 1500! And even if your peak is 1500 or even 1600 there are little chances that you will maintain 1500 for 3 minutes.
So the higher is your peak in a physical quality, the easier it will become to develop the capacity for that quality (capacity = maintaining a high output over a significant length of time).
If your deadlift 1RM is 500, then doing 21-15-9 with 225 will be much easier than if your 1RM is 315 because each rep will require much less effort and will thus fatigue you less.
So if you want strength-endurance, the first thing to do is develop and ample strength reserve.
If you want power-endurance you first need an ample reserve of power.
If you want speed-endurance you first need an ample reserve of speed.
They used to do it the other way around: work on the endurance "base" then work on getting faster over the required distance... but nobody does that anymore. Quality is much harder to develop than capacity and thus needs to be developed first.
I believe that earlier in the off-season you should maximize strength and power and this is done by training them by themselves using protocols that are effective at building limit strength and peak power. During that time the energy systems that will allow you to have good strength and power endurance are also worked, but by themselves with activities that do not require a high force or power output. For example a rowing ergometer, airdyne/assault bike or running/swimming.
We are talking mostly about the anaerobic lactic energy system... so an intense effort lasting about 60-120 seconds. We do bouts of that with medium rest periods.
We also use slightly less intensity over time periods of 5-8 minutes.
Then we gradually merge the strength/power and energy systems components.
At first we use complexes... for example a set of heavy lifting followed by a set of energy systems work.
5 deadlifts with 75%
60 seconds all-out on the rowing ergometer
3 minutes of rest. perform 3-4 sets
Then we switch the order (energy system drill first in the complex).
And finally we can use the strength/power movements for higher reps.
Each of these phases should last at least 4 weeks, sometimes up to 6-8 for some phases depending on the level of the individual.