I guess only you know what your ultimate goal is. That said, I like to practice to songs rather than exercises and sometimes (or a lot of the times) I have to slow down the original song to 80 or 90 percent speed to a speed that I can keep up with for the entire song. As I get more used to the speed, I start to increase it until I can play the entire song at normal speed. Examples of songs that you can use to practice this are:
1 Spheres of madness by Decapitated (it includes triplets)
2 Juular by Devin Townsend (important to notice how the snare is doubled on 1,2,3,and 4 while the hats are single hits)
3 Painkiller by Judas Priest (Iconic drum intro)
4 Under and over it by Five Finger Death Punch (just for endurance)
5 Laser canon deth sentence by Dethklok (Definitely for endurance and if you can play it at full speed you should have no issues playing triplets)
6 The Devil's name is surveillance by Messhuggah Notice the syncopated snare and hats vs the furious double bass speed..
7 Mandatory suicide by Slayer long double bass outro but I particularly like how Lombardo switched patterns in the first part.
8 Desperate cry by Sepultura (Iconic outro CRYYYYYYyyyyyyy!)
9 Road of resistance by Baby Metal (Harder than it sounds if you play it correctly).
10 Most of Unlucky Morpheus songs... (That drummer is just full of energy).
11 God of emptiness by Morbid Angel (slow song but the purpose is to play it super tight).
12 I almost forgot Hot For Teacher by Van Halen not particularly hard beside the intro but a ton of fun to play.
Notice all of the above songs have long continuous double bass parts and some (2, 5, and 6) are a test of stamina.
Bonus song (not as much double bass but this is to practice with your weak leg instead of your normal leading leg.
Lilith Immaculate by Cradle of filth. If you are right handed (and footed) try to play the single bass drum parts with your left foot, it is a workout and you get to be amazed that Marthus (the drummer) was able to keep that speed up for that long even with your leading foot.
There are tons more examples but I guess those should be enough. Now if you feel froggy, you can try doubling the speed (bass drum hits) of slow songs such as Lamb of God's Laid to rest and try to keep that speed through the whole song.
And one thing that hasn't been mentioned in this thread is that you got to get your legs stronger, and double bass practice alone is not fast enough. I personally quadrupled my speed by simply running. I don't like running, I was forced to run because of the military. I got in better shape and I was able to play any of those song with ease, then I slacked and slowed down but I am back to running regularly (and you don't have to run marathons just enough to make it worth your time) 2 miles or 5 km every other day will be more than enough... I don't know what your fitness level is and also don't know if you already do this but if you don't, you should consider doing it. most fast double bass players I know have some form of exercise included in their routine. Khrim (ex Decapitated) bikes everwhere, Rybalchenko from Fleshgod Apocalypse runs too and so do must metal players (if they don't run, they have some exercise routine (besides the double bass practice) also have you check Martin drum academy?
https://marthyn.net/ I have not tried his course but I will buy it once I get settled in my new home ( Military moves us every 3 years but this should be my last one). The price is cheap and I don't see him blowing smoke to people to tell them they can do it, he is realistic and he is quite fast himself. hopefully any of this super long response helped in some way.