How hard should my interval workouts be?

Pushing very hard in an interval session triggers a massive training response. A guide is to run shorter reps at about 85-95 per cent of your maximum heart rate, and longer reps at around 5K pace. However, pushing yourself near to your limits must be done at an appropriate time. If you’re new to interval training, don't run at full effort to begin with. Likewise if you’re returning from injury or illness. In the days before a race, it’s best to run intervals at a reduced effort as part of your final preparations.

How hard you run each rep depends on variables too, such as the length and number of reps and the length of recoveries. You’ll need to be able to gauge how much effort you put into each rep so you can run consistently throughout the session and finish strong.