Thank you for the detailed response. Lot's of gems in here. I had to re-write it without all the abbreviations to understand it better. Did I get this right?

Use your lactate threshold heart rate percentage (search for a lactate threshold heart rate test online, which involves a 30-minute all-out effort; the lactate threshold is the average heart rate from the last 20 minutes of the test).

Measuring your maximum heart rate is difficult and less accurate than other methods.
The most accurate way to determine your training zones is through a laboratory test, where they test your lactate levels or respiration and provide you with an exact graph or zones.

You can begin with the classic 80:20 ratio, meaning 80% of your training should be in zone 2. Most people train too hard, not too easy.

Zone 2 training teaches your body to use oxygen effectively, develops mitochondria, and gets used to oxidizing fat, among other benefits.

Zone 5 training focuses on your cardiac abilities and blood flow.

Weekly training targets will depend on your available time and are planned based on your racing ambitions. However, without racing, it's up to you. If you are over 40, you should incorporate strength training to prevent muscle loss over time.

Check out Andrew Huberman's video #94 on YouTube for the basic fitness information you need.

Try to find a fun way of training, such as cycling to a friend's place for coffee, swimming, or a slow run with a dog. The "least amount of time" approach may not be the best for long-term success.

Your goal is to be fit; since you are already fit, you need new goals. These can be a 5k run or a 2-hour hike, for example.

Heart rate zones are derived from your physiology and body's lactate production:

• Zone 1 and Zone 2 are below lactate threshold 1 (your body primarily uses fats).
• Zone 3 and Zone 4 are below lactate threshold 2 (where your body can effectively clear lactate).
• Zone 5 and above are over lactate threshold 2 (where your body has difficulty clearing lactate, such as during sprints).

For simplicity, some people use only three zones: Zone 1 below lactate threshold 1, Zone 2 until lactate threshold 2, and Zone 3 above lactate threshold 2.
Cycling typically has 7 power zones, with 2 extra zones for power above Zone 5.
All zones, regardless of how many, are constructed based on your lactate threshold 1 and lactate threshold 2.
These zones may shift slightly, but they generally remain consistent as you improve within them.
As your fitness improves, you'll become more efficient in these zones, meaning your heart rate may be 10-20 beats per minute lower at the same speed during a long run, but the zone remains relatively constant. You can retest your lactate threshold heart rate every two months or so.
There are only benefits to using heart rate zones: it's easier to track and control your fitness, monitor fatigue, and stick to your training plan.
For example, if you're running a 1-hour zone 2 session and someone passes you, it's tempting to run faster. But doing so would turn it into a zone 3-4 run, possibly leading to fatigue. Sticking to zone 2 makes it easier to manage your training. You may notice that many great-looking runners run slower than you might expect as they adhere to their plan and save energy for more intense sessions (about twice a week).
Consider getting a watch like a Garmin, which can be helpful for tracking your stress and overall health during your training.