I agree with what everyone has said here. I just have a couple of things to add.
First of all, I'm a lazy practicer. My cueball control isn't as good as it needs to be to run 100s. If you point at a spot on the table for me to hit, I'll get within a foot and a half on average. I've seen people practice position by laying out a dollar bill. That's the kind of control I think that runs 100s. I think getting into the habit of picking an exact spot on the table to land on is one we all need to ingrain. It's so easy to get lazy and sloppy, and all the time we are ingraining laziness and sloppiness. I think if we diligently focused on this while we played, we would learn more precisely where the cueball goes as a side effect. It has to sharpen us up.
Also,
If you can run out racks of 9-ball, you can most certainly run more than 20 balls in straight pool with the following mindset. I'm not suggesting it's the way to think if you want to run 100s, because you will get trapped at some point because the balls have to cooperate. But when I think this way, runs over 20 come very often.
Before you shoot any shot think of only one thing, "Where is the safest place to put the cueball so that I have more than one shot to choose from on the next play?"
You empty your mind of all but insurance. You don't save break balls. You don't plan your end pattern. You only want to be aware of zones that have options available. When you're down to about 4 balls left in a rack, you choose the best one available for a break shot. It may not be ideal, but if it allows you to touch the rack, it may be all you need to string a couple or a few racks together.
Running balls this way gives me easier shots to play. No heroics necessary in order to preserve patterns. There's also very little brain energy required, so you don't worry and focus only on shotmaking and easy position.
Since the shots and the position play are going to be easy, you can be confident that there's no reason not to run a couple of racks just based on odds alone.