How to approach zone practice if heart rate zones are completely inaccurate

You also may just be especially fit. For example, Dale Stone, one of my favorite MTB YouTubers, has a max heart rate of 208 or so, and is still capable of talking at 180BPM and higher. Heart rate guidelines are just that: guidelines, and there's no guarantee that you fall within them.

I take it you know or feel what your optimal cadence is. If not, somewhere between 70-90 rpm usually is a good starting point to experiment with.

Find a gear in the back while on the big chain ring that you can peddle at between 70-90 rpm and that you see your heart rate stay flat for the duration of your ride. Again a good starting point for a new rider is 10-15 miles and eventually growing that number to like 20-25 for a short ride. If it continues to increase to your max, you are pushing too hard of a gear so shift down one cog.

Keep doing that until you can ride 25 miles in that gear without your heart rate reaching zone 5 or let's just say for you about within 15 to 20 bpm of your max heart rate.

If you want to go faster, now ride 50-75% of the same distance with the next smaller gear and try to keep your cadence above 65 rpm. You will notice your heart rate rising, but that is just helping your body adapt. You will also feel your muscles hurt more, but that is again adaptation.

Keep doing that until you are cruising around at 28 MPH and then go buy some wheels, kit, aerobars, 53 tooth chain ring, etc.

Hopefully that helps and always make sure your heart is healthy enough for exercise and all that other liability stuff.

In regards to your heart rate. Maximum heart rate is different for many people because there is a combination of factors, and those most important factors are the age of individual and size of the heart.

As we age, every year your avg/max HR goes down by about 1beat per min. With the size of heart, smaller heart achieves higher max HR and opposite. HR also changes based on fatigue, dehydration etc. note that having higher max HR then other people is no indication of better fitness.

There are many heart rate zones standards because many people have different approaches and methods to training. Choosing heart zone standards will depend on what training program or workouts you follow. You can check these training zones that have been working for 100% of my athletes.

However if you don't follow any structured training, what you need to focus on instead is your heart rate threshold (or power threshold, if you have power meter). This is the most important metric when determining the training zones and your training. You can determine it by doing 30min full effort and last 20min take avg HR. If you have power, you can estimate your FTP here and check your cycling level.

Once you know the threshold HR, then all training should be based on that. Each training day you should focus on a different aspect of your fitness, so, for example, Tuesday focus on zones that are below your threshold, wed you can focus on at threshold HR and slightly over, and Thursday can be super intense above your threshold 120%.

Lastly, your threshold heart rate can change with your fitness, so make sure you monitor this at least every 2 months.

Max HR isn't a particularly useful metric. Not only is it particularly difficult to measure/estimate, but it can't be used to accurately define training zones, as there are large variations between individuals with regards to % of MHR they can sustain for various durations. It can even change for an individual based on their current state of training.

Basing zones on Lactate Threshold Heart Rate (LTHR) is a much more useful metric for determining appropriate training zones. The most common way to determine LTHR is by performing a 30 minute solo time trial as if it were a race, and taking the average HR for the last 20 minutes of the effort. https://www.trainingpeaks.com/blog/joe-friel-s-quick-guide-to-setting-zones/