Aerobic training focuses on strengthening the heart and lungs to provide a stronger base (i.e. aerobic engine) for riding longer distances without succumbing to fatigue easily. What this means for us riders is that we will be able to ride for a longer duration without having to stop for rests as frequently in order to catch our breath.
Keeping a high enough cadence (e.g. 80 rpm minimum) is crucial to effectively train our aerobic engine. Cadence is what will put our heart and lungs to work, so it is important to ride at the appropriate cadence that is optimal for training our organs. To attain the desired cadence, we need to select the appropriate gear ratio that is suitable for our current physical state as well as the road condition we are riding in. For example, a beginner cyclist may have to use slower gears to ride at the same cadence as a seasoned cyclist with faster gears. The seasoned cyclist may be riding at 30+ kph speeds while the beginner may need to drop the speed down to 20+ kph even though they both may be riding at 80 rpm cadence. Riding a flat road, uphill, or downhill will also require us to use a different gear ratio to maintain a consistent cadence.