Muscular endurance is pretty exercise specific. You'd think being able to run a long time would allow one to go long on a stairmaster or vice versa. Nope. It helps (certainly with the cardio aspect it helps a lot), but in the end you get better at a given exercise by doing that exercise.
So, you wanna improve playing endurance? Play a lot. This also helps other aspects of musicianship you won't get from hand exercisers.
One extra thing I do while in the car is lightly hold part of the steering wheel with my left hand like I would a guitar neck, with only thumb & all fingertips touching. Then I do mock "trills" with each finger until it gives out. It only takes a few minutes for each. All the while, incidentally, I am driving with my right hand (just wanted to make that clear!).
But mostly, just play.