Assuming there is no wind, the ball must have a minimum exit velocity to travel a given distance. This assumes the ball exits at the optimum launch angle. If the angle is not optimum, the velocity will have to be higher to achieve that distance. In that sense, distance and velocity are related.
I feel like pitching velocity is improved by long toss when long toss alleviates some limiting deficit in the player. That might be shoulder mobility or timing or sense of effort. I also believe that varied throwing stimuli help the player improve command of throwing in general. In our experience long toss, has definitely helped to improve velocity. We also have empirical data that shows higher velocities for any given day when long toss is incorporated as part of the pre-pitch routine.
The biggest long term increases in velocity for our older player have occurred sometime after significant general strength improvement and mechanical changes.
It is a little hard to discern what has had the biggest impact because frequently a continuous shotgun approach is employed where our player conditions, long tosses, uses weighted balls, improves mechanics, improves nutrition, and naturally grows. Doing all those things will have a big impact on velocity for most players. However, he likes to long toss and likes the long loose feeling that long toss gives his arm.