is it just a TechPro barrier? I hope there’s no concrete behind :O

In my opinion, something is wrong with your Garmin Zones.

How is your garmin calculating Zones?

Garmin fitness devices assign heart rate zones based on your maximum and resting heart rates set in your user profile

so check that Max. and Rest HR are correctly set and change them if needed

My best guess is your Maximum HR is set wrong (too low). Look in your training logs what’s your max HR in some intense activities, and considering some unaccurancies you can make a good stimation of your Max HR. This applies IF you do some intense runs/exercise. You need to go hard for some minutes. At your age, unless some unlikely conditions, you should reach about 180 bpm.

MAF (maximum aerobic fuction) HR (heart rate) is NOT in your Z3. You should NOT be over that number (if you want to maximize aerobic function), it’s not an average value for your run, it’s the top limit, so your average HR for the run should be clearly below that number (obvious). You may walk in part of the route if it is too steep or you go longer than usual.

MAF is NOT Z3. I’ve not seen anywhere that.

“I considered…”

well, you (or I) could consider whatever zones :D

usually “zones” are 1 to 5, being 2 considered easy, conversational, nose breathing, … pretty similar to MAF but not the same, being MAF more restrictive on the top limit. Do not cheat when calculating MAF, usually just 180-yourAge.

yes, you can set garmin to use HR zones as target

you can manually edit garmin HR zones to whatever you want (correct or not)

According to the MAF formula my low heart rate zone ends at 142bpm

180 minus Your Age is 142. I find MAF heart rate very accurate (considering breath pattern, perceived effort, etc.). This “142” value is “the maximum HR you should run at”, not “your average HR for the total run”. You have to keep it below that number for the entire run.

Z2 is also a calculated value given your Máximum HR (ex. all in effort in the hills), age and fitness level. If garmin says a Base run at 138 then it’s pretty similar to MAF, so just focus on run relaxed, easy nose breathing, and keep an eye to avoid being over 142 on the steeper zones.

Zone2 is where one should focus to increase areobic function and where you should spend most of your time (for endurance and general health targets). Eat clean. Try to balance exercise with the rest of your life. Go hard once every other week.