Training Long Term

I've been swimming masters now for well over a decade and I've hit a bit of a lull. Last year I set my Go The Distance goal at 550 miles and missed it, so this year I revised it downward to 500 miles and as of today I'm 66 miles behind the pace I need to be at to hit that goal. I just don't have the desire to get to the pool as often, or stay in as long, as I usually do. With that introduction my question really is: how do YOU train long term? Do you try to stay consistent or do you vary from year to year? Historically I've been very consistent, but I wonder if intentionally varying things is a better long term strategy.