Aerobic fitness doesn’t change overnight.
It builds gradually through repeated exposure to the right kind of stress — mainly consistent, low to moderate intensity work.
The challenge is understanding how much is enough to improve… and how long it takes before you feel and see the changes.
Minimum Effective Dose vs Optimal Volume
You don’t need hours of cardio every day to improve.
The minimum effective dose for many people is:
• 2–3 sessions per week
• 20–40 minutes per session
• Mostly in Zone 2 / conversational intensity
This can begin to stimulate improvements in heart efficiency and mitochondrial function.
The optimal range for more noticeable and sustained improvements is often:
• 3–5 sessions per week
• 30–60 minutes per session
• Mostly low to moderate intensity, with occasional higher-intensity work layered in
More volume isn’t always better — it’s about finding the most you can recover from consistently.
For more info on how Morpheus determines your weekly training zone targets, check out this article --> https://8weeksout.com/2023/05/14/how-much-volume-and-intensity-do-you-need/
Timeline for Aerobic Adaptations
Aerobic fitness improves through structural and metabolic changes.
Within 2–3 weeks
You may notice workouts feel slightly easier, but most changes are still neurological and efficiency-based.
Around 4–6 weeks
Mitochondrial density begins to increase more noticeably, improving your muscles’ ability to use oxygen and produce energy.
Around 6–12 weeks
Stroke volume (how much blood your heart pumps per beat) improves, which can lower heart rate at rest and during submaximal exercise.
These deeper changes take time, which is why patience is essential.
Why Progress Feels Slow at First
Early aerobic improvements are subtle.
You may not feel dramatically fitter right away because:
Heart and muscle adaptations develop gradually
You’re building internal efficiency before speed or power increase
Fatigue may temporarily mask fitness gains
This phase is foundational — it sets up bigger improvements later.
HRV and RHR Signs Aerobic Fitness Is Improving
You may not see performance jumps immediately, but recovery markers often change first.
Signs aerobic fitness is improving include:
• Gradually lower resting heart rate
• Faster heart rate recovery after exercise
• More stable HRV trends over weeks and months
• Smaller HR increases at the same pace (less cardiac drift)
These signals show the cardiovascular system is becoming more efficient, even before pace or power changes dramatically.
Consistency Beats Intensity
The biggest driver of aerobic improvement isn’t crushing one workout.
It’s repeating manageable sessions week after week.
Large spikes in volume often lead to:
Excess fatigue
Suppressed HRV
Inconsistent training
Steady, sustainable volume leads to real long-term gains.
The Big Takeaway
Improving aerobic fitness doesn’t require extreme training, but it does require consistency.
A few weekly sessions can start the process, but meaningful changes in mitochondria and stroke volume take weeks to months.
Progress may feel slow at first, but HRV stability and a gradually lower resting heart rate are early signs your engine is getting stronger.
The goal isn’t to rush it.
It’s to build a system that keeps improving for years.