OK so in general, if you are doing something, practice it more and you get better. No one would take issue with that - the problem is whether or not someone is doing a particular thing or not. If not, practice won't improve that particular thing. First you have to start doing it.
If no one teaches it, then all we can do is try to imitate. Problem is, we may think we are doing a good job imitating but we are doing it totally differently. (In other words, "local muscle control" versus "global body control" that utilizes the tanden and a whole body's worth of developed connective tissue.)
At any rate my comments only apply if you think you may possibly not be doing it already. I don't know why I initially had that feeling 5 years ago, but upon investigation I became convinced.
But yes, if you are already doing it, then putting in the training time is all you need.
You get good at what you're doing whether or not it's a "good" performance - it has to be directed or "deliberate" practice - that means learning the actions that constitute "good" performance in whatever it is. If you're training a "bad" movement skill over and over again you get really good at doing that "bad" skill, which is unfortunate, and a severe waste of time. I've seen athletes with amazing physical tools struggle for years to overcome poor technical skills that were developed - extremely well developed - early in their sport career. You need to be lucky when you start, to find someone who helps you learn "good" skills so you don't have to fix them.
Training is also specific to the activity being trained. A GREAT judo athlete may not be great at gymnastics or Kempo or Kali, but they have an athletic ability that will assist them in learning the other activities, so they may become better at the new activities than someone with no former background in sports/MA. A GREAT aikido person may or may not be any good at (say) target shooting but they have a good chance to develop those skills because they know how to focus, train, relax, and so on, perhaps more than a couch potato who really likes twinkies (which are still made in Canada, BTW, I just don't know what they taste like).
I don't doubt that IS training is valid; I don't know if I have "IS," whatever it is, but if I do, whatever I have hasn't been called that. (I probably don't, fwiw). Ennyhoo. If I can get my knee working (the surgeon's office called today and postponed the consult.. Grrr.) properly again I'll see how much I can do about getting back into aikido and then perhaps will seek tutelage in IS... My curiosity is piqued.