It seems like being told what to do would make physical fitness easier. "Pick up that weight and do ten reps. Then put it down and run on the treadmill for one lap."
On one level, it does make it easier because there's no guesswork involved. But it doesn't make the physical task any easier. Even guided drudgery is still drudgery.
What makes this better? Is it merely a result of your trainer's training style?
Maybe. But the real problem is often not the programming itself. It's your level of engagement. When we're engaged in a task, it's more enjoyable and more rewarding, and time goes by faster.
The question is, what should I be engaged with while exercising? And more importantly, why? All I'm doing is physical exercise. I move my arm; it bends. My body does it for me; what more do I need to to?!
That's one way of looking at it. Of course, another way of looking at it would entail viewing each physical movement, like a curl or a squat, not as something that happens automatically, but instead as a specific task requiring skill. Each task can be done skillfully, or it can be done poorly.
When an exercise is done poorly, three things happen: 1) the exercise becomes ineffective, 2) the exercise becomes unsafe, and 3) the exercise does things it's not supposed to do.
For example, take a bench press movement with the elbows undesirably flared way out to the side. The movement is no longer predominantly focused on the chest, so it becomes ineffective. The movement places undue strain on the rotator cuff, so it becomes unsafe. And lastly, with the elbows in that position, the movement strengthens the internal rotator muscles of the shoulder, which is it's not supposed to do. See what I mean?
How is this avoided? Listen to your trainer when he or she tells you which muscles you are supposed to be feeling, and if you don't feel them, say something! And if you're not sure, ask!
This will allow you to do something very special and valuable. It will allow you to engage with the exercise! Your mind and your muscles will be connected. You will become aware of the individual movements and sensations that are supposed to accompany each exercise. You will feel the muscles working and the positive feelings they elicit.
From that point after, your workouts will never be the same. Gone will be the passive drudgery of just waiting for the clock to tick down. You will know how to execute each movement effectively, and go home knowing you got the most out of each one.
Very little of substance was ever accomplished by accident. That includes healthy muscles, bones, joints, and heart. If exercise feels like a passive activity to you, there's a good chance you could be getting more out of it by simply connecting your mind to your muscles.
Your trainer can't do this for you. Only you can decide it's important to you to make the most of your time in training.
Your mind and body are the only ones you have. It's best if they work with each other, not against each other. Then, everybody wins! And that's what we want :)