Easy days easy, hard days hard
Most of your training should feel genuinely easy, with a small dose of real quality. The common trap is spending every session in a grey middle that is too hard to recover from and too easy to drive big gains.
The grey-zone trap
Left to instinct, a lot of us train at one speed: moderately hard, most days. It feels productive, but it slowly wears you down while rarely being hard enough to lift your ceiling. The fix is to make your easy days properly easy, which then lets your hard days be properly hard.
Mostly easy, a little quality
Keep the bulk of your time in the easy band, conversational, under about 70% of max heart rate. Add a smaller amount of tempo and interval work for the sharpening. See the exact bands with the heart-rate zone calculator.
Easy should feel almost too easy. If you can barely hold a conversation, you are in the grey zone, not the easy one.
Why it works
Easy volume builds the aerobic base that everything else stands on, at a cost your body can absorb day after day. The hard sessions, kept few and genuinely hard, provide the stimulus to climb. TrailFree writes each session with a plain effort cue for exactly this reason.
Find your bands with the heart-rate zone calculator.