“Hard fun.”

In my opinion, this two-word phrase constitutes the ideal description of an endurance training program that’s really working.

As a coach, I can’t think of anything I would rather hear an athlete say in response to the question, “How would you describe your experience of the current training segment?” than “Hard fun.”

Why? Because an endurance training program must be both hard and fun to be optimally effective, and if it is both hard and fun, there is nothing more that it should also be.

The job of any endurance training program is to improve your fitness by the maximum amount possible within a certain span of time. An endurance training program can’t fulfill this objective unless it features a number of workouts that test your current limits and an overall workload that does the same. And a training program that does these things will be experienced as hard.

At the same time, though, emotions are important in shaping the outcome of the training process. Numerous studies have shown that the more an exerciser enjoys an exercise program, the more likely he or she is to stick with it. By logical extension, the more an athlete enjoys a training program, the more he or she will put into it and get out of it.

Obviously, there’s a chicken-and-egg factor to consider. When the training process is going well (i.e., producing good outcomes), an athlete will tend to enjoy it for that very reason. But, without question, it works the other way around as well—that is, when an athlete is enjoying the training process for reasons that have nothing to do with outcomes, the outcomes will tend to be better. To my knowledge, this has not yet been shown experimentally, but studies have shown that people tend to work harder and perform better in individual workouts they enjoy more.

How hard a training program seems to an athlete is largely a function of what’s known as physical loading, or the volume and intensity of the workload the athlete is subjected to. How fun a training program seems to an athlete is largely a function of what French exercise physiologist Bertrand Baron refers to as affective loading, or the balance of enjoyable and unpleasant emotions experienced in the training process. Factors such as an attractive environment and compatible teammates can make any given pattern of physical loading more enjoyable for an athlete, reducing affective load and improving outcomes. But it’s also important for coaches to consider the affective effects of physical loading itself when prescribing training.

Specifically, training should be prescribed in such a way as to ensure that the athlete feels good most of the time—not all the time, to be sure, but most of the time. And throughout the training process, training should be adjusted based on the athlete’s emotional response to what’s been prescribed. If this sounds rather touchy-feely and unscientific, know that a 2015 review by Australian researchers found that athletes’ subjective ratings of their well-being were better indicators of their training status than were objective measures such as heart rate and blood markers.

The take-home message? To get the best results from your endurance training, make sure the process is hard, but never so hard that it isn’t fun. At the same time, make sure the process is fun, but never so fun that it isn’t hard.