Obviously, a well-regulated militia is one that is trained well.
Why is that at all obvious? Well-regulated could also mean certain specifications for their weapons. It could mean physically fit and a clear assumption that they know how to shoot.
The definition of "regulated" is as ambiguous and multi-faceted as "militia". Whereas I am convined that "militia" is essentially everyone. I still am not sure what "regulated" actually implies. It isn't there for fun. It is not like the 2nd amendment is an exercise in verbosity. It has a well thought out place and meaning.