I feel the need to explain my understanding of muscle physiology and exercise.
Muscles are made up of 2 fibre types, slow twitch and fast twitch of varying proportions.
Slow twitch fibres:
They are smaller than fast twitch and weaker.
They are made up of tiny mitochondria fed by tiny blood capillaries and burn mostly body fat for fuel. They need plenty of oxygenated blood to burn it.
They are the first fibres to be recruited when exercising until they cannot perform the task when fast twitch fibres take over.
They are the endurance fibres, slow and steady progress.
When specifically exercised, they grow more mitochondria over time, to produce more power.
To specifically exercise them, we need to have the intensity low so as not to recruit fast twitch fibres which means our heart rate will be low.
Fast twitch fibres:
They are larger than slow twitch and stronger.
They use ATP stored in them and the liver at first for fuel but run out after about 2 hours, this means we have to keep eating to fuel them for longer periods.
They can work without oxygen for short periods (anaerobic).
They are the workhorse fibres to sprint or get you up that hill etc.
When exercised, they grow in size and use more ATP.
To exercise them we need to increase the intensity to recruit them which means our heart rate will rise, switching from fat burning to more ATP burning, and more eating.
Most cycling is endurance and therefore should use the slow twitch fibres predominantly, sparing the fast twitch for the hills etc. It follows then we should train mostly the slow twitch fibres which, in turn means long slow rides at low intensities with a low heart rate.