What's the best metric to use in training?

Coach Karel & Xavier both prefer RPE (perceived effort from 1-10). Here's why.

It forces you to get to know yourself and to feel the intensity you're running at. But we're not going to lie, it takes practice.

Starting out, it's easier to follow a pace or HR, but as you practice more you start to feel the different intensities better and better.

HR depends on how you sleep, temperature, everyday stress levels, etc. The same goes for pace. Your threshold pace one day is not necessarily your threshold pace the next day. That's why it's important to learn what your threshold pace feels like.

The better you get at feeling the different intensities, the more effectively you'll be able to train, and the better you can adjust training when you don't feel the right trends in fitness.

It's also essential if you want to be able to run a strong race. But that's a whole other post.