To be a good trainer you need the facilities, not for example, going through the ridiculous process of arriving at the local park, having identified where the sprinkler heads are, anxiously keeping an eye out as your mate gets a few hundred yards away and hoping that no one turns up with their "mut" either on the lead or worse still running loose. These memories are so vivid. When dogs are young and they have ability they are generally relatively easy to train but as you have indicated, as time goes on, they become harder to train as injuries take their toll. This is where the "cream rises to the surface". When I got my first greyhound, a virtual giveaway ($40), I won a couple of races early on with her and I remember saying to my dad, quite naively, "I think I can now call myself a dog trainer". What "crap", she could have been so much better, no champion that is for sure, but a lot better.