I’m guessing that you are specifically talking about the list for Training Load Priority. If this is the case, you can manually select edit the load for an activity:
You could also automate this, but you’d have to create rules to differentiate between the types of run you want to identify. That should be easy enough for just easy runs / interval sessions. This can be done with scripting with custom activity fields.
You’ll also need to take a peek at the server-side data model for scripts. I think you’ll essentially need to change the icu.activity.icu_training_load attribute.