No doubt long distance practice is beneficial, but the subject of the thread is:
"What distances should I be practicing SD shooting at?"
I'm going to throw another twist at you more important than the range and that is how you shoot. I drill at various ranges from 8 yards to 110 yards because that is how my range is set up. The important thing is how you shoot and it should be mixed up with standing, kneeling, sitting and prone. Some exercises should be strong hand, weak hand and two hand. slow fire and rapid fire, shoot from a chair and shoot from behind a barricade. I know that's hard to do on a public range but if the range isn't crowded and you talk to the range master you might get some of those drills in. If nothing else get the strong, weak and two hand drills in, you never know when the flag goes up what hand may be available to you due to injury or some kind of encumbrance or the position you are in.