What works for me is that I like going to the gym just to go to the gym. More specifically, I love the feeling of being able to lift lots of weight and I love the feeling of being sore the next day. This isn't necessarily changing an instrumental goal into a terminal goal, but it does sort of reduce the distance between the original terminal goal and the instrumental goal. The original terminal goal (getting fit) has been moved closer in "goalspace" to the instrumental goal (lifting at the gym) by transforming into the terminal goal (get sore).

So in general it might be easier to think of shifting goalposts as far as terminal/instrumental goals. Picking a terminal goal that's "closer" in goalspace to your original instrumental goal might make it more manageable.