I do find outside workouts provide some “training” that’s less obvious than hitting numbers. Like what it “feels” like to hold a certain effort level across varied terrain. How to resist letting the terrain dictate when you rest. . . e.g. a slight decrease in incline or descent letting your power drop by 1/2 without noticing.
I’ve thought of MTB, since typically those routes have more uninterrupted climbing. But I don’t have power on my trail bike. I’ve been using flats there, so uncertain if it even makes sense. Particularly with how variable the power effort is. . . . like you’re basically at 0 power when you’re defending, but that doesn’t mean you’re not working.
HR seems to capture my MTB efforts better.