I've noticed for a long time how the fitter players have to work harder to tire themselves out, that's obvious though, but also recover from their exertions much quicker, that's not so obvious until you look for it.
That's what I'm seeing now that I've been playing regularly for a couple of months. I haven't really trained hard but my fitness as improved and I can really see it in any activity I do simply because I recover much sooner. I get my breath back sooner and any pain I feel quickly subsides. I'm also ready for another burst very quickly and can keep pushing myself for many more bursts of activity.
That to me is the example of a training effect and seems true whether its mental or physical training you're doing. So as a general rule I find the simplest way of training is to use really intense bouts of activity with short rests. Do this regularly depending on the amount of recovery in between and the impact and strain of the actual activity you can do this daily or every few days even just once a week and you'll find great improvements from very little work.