Swimming does not create speed. Read up about fast twitch (speed) versus slow twitch fibers (endurance) and then look for programs for each. Michele Smith swears by plyometrics.
Explosion is often more important in softball than endurance, while endurance is useful for an active healthy life, a far more important goal. However, watch boys versus girls. We need to get our girls to do what the boys do for explosiveness. Our pitchers do it, so the other players can as well.
What do the boys do for explosiveness?