Heavy hitter reporting in to the rescue.
Bad news first: If you really wanna go this route, there are absolutely no shortcuts, tricks, magic spells or whatever to help you.
This way of playing is a long term goal with no instant gratification!
Also we leave the territory where music is supposed to be fun and enter the realm of mindless repitition,muscle soreness, and brain eating bad TV shows.
That being said, how do you actually build stamina? To answer this I'd like to change the view a little, away from music and take look to sport especially decathlon. The reason for that is, that for this playing style we really have to think about how our body works since it has much more in common with sport than "regular" bassplaying.
Generally when we think about sport and try to categorize it, we divide it into two categories, first strength-based sports like Weightlifting and second endurance-based sports for example Long-distance running.
So if look at what the physical requirements for strength-based sports are, we find that more muscles equal better results but what for endurance based sports? Again it's muscles!(Although, you dont need so much of them) So what is the difference?
(By the way, just for you smart-asses who read along and think that I'm oversimplifying things and that this or that factor should also be mentioned: My aim here is not to write a master's thesis on sports, but to give a guide to a certain style of bass playing! Now read on.)
The difference is the way how your muscles are trained. For strength-based sports, you use a training that allows you to mobilise a lot of strength in a short period of time. For endurance-based sports, on the other hand, you use a training that allows the muscles to mobilise a much smaller amount of power for a very long time.
This is were decathlon comes into play. Because for decathlon, athletes have to train both their strength and their stamina/ endurance. So while decathletes are not as strong as specialized weightlifter, nor as fast as a specialized runner, they will show a much better over all performance in different disciplines than someone who is very specialized.
So how does this help with fast playing and hard hitting?
Well, if we look to usual (already mentioned) given advice on how to get faster, we are told use less strength and relax/loosen up. This is not a bad advice in the first place, but the consequence is, that we train our muscles in very specialized way, much like a long distance runner.
However looking to decathlon we see, that it is indeed possible not only to be fast and enduring, but also to be strong.
So what we need is a training plan that resembles more of a training plan of decathlet, than a long distance runner. (This is the part where we enter the realm of mindless repitition,muscle soreness, and brain eating bad TV shows.)
Here is what I do, basically my after "practice" workout: First (or rather zero, cause this is the "practice" part, not the "workout") I do my "musical" exercise. Scales, songs, technique. All the stuff that requires major attention and concentration.
For me it's necessary to have done this stuff first, cause later in the evening when it comes to "workout phase" I usually don't have enough mind power left, to concentrate on these things.
The actual "workout" begins, when I start watching TV. (Around one or two hours after "practice"). As I watch (I like Anime), I take my bass (not plugged in) and play straight eights.
Just on one, open string for... maybe 2 or 3 minutes. Than I go to next string. However I make sure that I play with the desired sound/ strength. I also try hold a steady tempo throughout the episode as I watch. (For those of you who are not familiar with anime, an average episode takes about 20 minutes).
In order to keep tempo, I have to start way slower than the tempo I want to reach. Usually it takes around two weeks, then I start to feel comfortable with this tempo (no more tensioning or slowing down) and I raise the tempo by 5 to 10 bpm. Then repeat. (During this time I may also add triplets and gallops, but no complex Rhythms)
Over time you will build up speed, stamina and enough strength to get your desired sound.
Some more things to consider: Dont hurt yourself. When you start out, and you feel tension or pain in your arm or fingers, stop. Take the Bass away for a few minutes, then start again. Slower, with less strength. If there are still problems, take the bass away and come back the next evening.
This whole process will take you a long time to get you where you wanna be. Just to give you a scope: When I started out with this kind of "workout" to bring my right hand in shape (around 25 years ago),it took me around 6 month to get to point where i felt i had made some noticeable improvement. For the first 3 month it seemed to me that i make no improvement at all.
As in all sports, you lose muscles faster than you gain. So you have to be in constant training to stay in shape.
So that's basically all i have to say. Have fun and good luck.