By training close to the MAF heart rate but still below it, you need less training time to get the same TSS compared to zone 1. For example, doubling the power output means you need one fourth the duration.
Why do professional cyclists still prefer lots of zone 1 training when recovery rides don't need to be long? In polarized training, there's lots of zone 1 training.
MAF heart rate Subtract your age from 180 to get the heart rate. Train between that heart rate and 10 beats below. The closer to the calculated heart rate, the quicker the aerobic base will be developed https://philmaffetone.com/180-formula/