You can already play really fast in an uncontrolled way by just letting your foot twitch out. With some fooling around and sounding bad, you can gradually gain some control over that so you can use it musically.
I also suggest working on audible stuff first, deprioritize inaudible stuff.
Like everything that you can just naturally do, you got to start slow and increase speed as you get used to it.
simple exercises are:
RRRR LLLL repeat for about a minute at the fastest speed you can play it clean (by clean I mean the volume of your hits should be pretty even).
Then do
RRRRRRRR LLLLLLLL for another minute
Then RLRLRLRLRLRLRLRL RLRLRLRLRLRLRLRL Again as fast as you can play it clean.
Take some rest you don't want to feel pain just a little burn but don't push it too much.
Repeat a few times to accomplish between 20 and 30 min, call it a day.
You of course can play with whatever technique is comfortable to you Heel up or heel down etc.
And you can make any variation of the exercises you want as long as it is fun for you.
Finally find a slow song with double bass and try to play the bass at double the speed. for example Lamb Of God's Lay to rest. very easy song but not so easy to play at double the speed.
or find a slightly faster song to practice stamina Such as Decapitated's Spheres of Madness.
Then after a month or two you should have goals:
To be able to play Juular by Devin Townsend :
or Morbid Angel's Dawn Of The Angry:
or if you really want to test your stamina this song: meshuggah the demon's name is surveillance:
or this: