I am the owner of a 33 year old Perkins 4-154 with 9600 hours on it. When I bought the boat it had 4600 hours. I have never cleaned/changed the injectors or adjusted the valves. I change the fuel filters only when the engine begins to hesitate. I overheated it once and blew a head gasket. I do change the oil every 100 hours. The engine now leaks oil into the bilge, so I have to remember to turn off the bilge pump as I enter a marina.
I'm not proud of the fact that I do so little engine maintenance but the engine kicks over immediately every time I turn the key and I get the same 3/4 gallon per hour I have always gotten and still cruise at 6.5 knots at 2000 rpm.
I know repowering is in the near future and I'm not looking forward to the decision to go with a current marine diesel or a rebuild for exactly what Bill R. was talking about.
In addition to ASA classes, you should also look into the USPS offerings. There is probably a local squadron you can join.