What can I say. Their 'internal' seems to just be waxing poetic about regular old external strength.
So internal is simply relaxed external?
No, trust me, your in for a very pleasent surprise...one day.
Keep training that's the first thing. Secondly, start working on strengthening from the inside out, (pilates focuses on this sort of thing whilst I don't think it is really 'internal') balance is more important than brute strength, the two together are of course optimumal. These two factors will leave you in good stead for the future. It is a proven fact in physical therapy that having good internal strength will protect your back and balance your whole the body, reducing the risk of injury. This is not real internal skills that Tohei teaches but a good start.
Anyway that's just my view.