We’ve all been there: You head out for an easy run and settle into your usual pace, but after a mile or two, your breathing feels labored and you notice your heart rate climbing higher than normal. However, you’re still going at your usual pace on your usual route, so what gives?

If you judge the run by pace alone, you might see no reason to change anything. But those other signals suggest you’re paying a higher price than usual to hold that speed.

Of course, you probably already know that factors like heat , hills, fatigue, and stress can make your usual pace feel harder. But with so much advice about what to pay attention to during a run, it can be tough to know which signals to trust and how to use them to evaluate your effort.

And without a clear way to interpret those signals, you might inadvertently push too hard on an easy day or miss the intended effort during a speed workout.…