Here's the thing. I come from a classical piano background. That means 6 times a week, I practiced 3 hours. About 1 hour was on scales and arpeggios. You'd think I'd have developed great technique. And largely, I did. But it's not as though my technique got indefinitely good. I didn't automatically get better every week, but over the years, it got slowly better.
It's been many years since I regularly played piano. I haven't legitimately practiced scales in many years. But you know the crazy thing? I can't do the scales quite as accurately as I used to be able to, but my top speed hasn't really changed. And so, my usable musical speed on piano is still about the same, because I can still think at the same speed. Other than a few more mistakes, my technique is still pretty solid. Though I stopped getting obviously faster, I haven't actually gotten noticeably slower. And because I'm older and wiser and my ears are better, I actually think in a lot of ways, I play more expressively and emotionally and get more mileage out of my raw speed than I did in my younger days.
Now, if this were weight lifting, if you stop lifting for 10 years, you won't be able to do the same kind of weights you used to be able to lift. So why is piano not like that? It's because music is not a sport.
Finger dexterity in music is not the same as athletic performance. People treat music too much like a sport, and it's not. It's more like solving a puzzle using your mind. Your brain can't grow faster just because you work your fingers harder. You play faster because you get smarter, not because your fingers get more athletic. If you play efficiently, you barely need any strength to play bass. Just about any 11 year old has the finger strength to play bass. The rest is how efficient you are with it. And that means it's your brain doing the work.
And that also means, if you feel pain, stop. If your fingertips hurt, sure, maybe they just need to toughen up, but for the large part, pain just means that the instrument is not set up properly and/or that you're not playing efficiently and you need to attack the puzzle in another way. I've been playing regularly for years and I don't even have calluses, so it's not even clear that fingertips need to hurt. So stop stretching if it's painful. Find a better way to play. Make small sliding motions instead of stretching from fret to fret.
Don't expect things to happen overnight. I often find that once I've begun working on something, my brain just needs time to think on it. Visualize playing the scale in your head. You don't need an instrument to do that. You can do that 10 times in a day and you're growing your ability to play that scale. Music is not like sports training where running 20 miles gives you more cardio than running 10 miles. Because you're trying to grow your brain, there's almost no point in spending 40 minutes doing something you can already do well. Even on classic piano, where each scale looks different, I would run through each scale maybe 4 times (4 octaves). Then move on. It's more important to find your breaking point and practice something that's difficult for you, rather than to keep doing something that's easy. Like I keep saying, it's not a sport. Finding your breaking point is what forces your brain to learn to be more efficient.
Some instruments do demand a little bit more physical fitness, that's true. Certainly playing metal drums is like running in place, that can't be helped. But bass is an instrument of dexterity. And honestly, so are drums, even though it might require a little bit more physicality. Otherwise, you wouldn't have great drummers who are out of shape.
