The training rod is definitely not a new addition. I used it in my first game a couple of years ago because I found fishing too hard. There was a dialogue where Willy asked me how I was finding the fishing, and one of my options to reply was “it’s too hard”. Then he told me to buy the training rod. I don’t know if it was available before that; I never looked. But try talking to Willy when you see him and maybe it will come up?

The lake outside the mine is a good place to start. You get carp there, and sometimes they never leave the position your bar starts in, so you don’t even have to do anything to catch them - free XP!