The watch doesn't actually measure your heart rate. It measures light. You can see it has these LEDs blinking on the back. They project light on your wrist and then a sensor tries to detect movement there that looks like pulse. As you can imagine, whenever your wrist is not stable, like when running for example, this whole process gets a bit messy because wiggling of the watch is easy to confuse with the pulse. Additionally, external light that gets between your wrist and your watch will also interfere.
You could use a chest strap and it should be not just more accurate, but more reliable and have lower latency, higher resolution. But using the heart rate data is actually pretty tricky.
Personally I do interval workouts that are based on paces, not HR. Then threshold on effort and pace, just having a look if the HR is not too off. Strides purely based on perceived effort. Easy and long runs based on whatever feel easy, but calibrated based on what I know my easy Geary rate and easy pace should be (1:00-1:15 min/km slower than 10km race pace). So at the end, even though I taka a HRM on every run, I don't really rely on HR data that much. It's nice for seeing things like maybe HR is a bit higher today and I don't feel great so there's something to it, or remind you take it easy when you forget yourself for a moment, etc.
But if you were to use HR data as guidance to your training, it only makes sense if you (1) make sure that the data is reliable (meaning a chest strap); (2) calibrate zones properly using tests; (3) have an understanding of limitations like latency (you HR even if measured perfectly takes time to catch up when intensity changes - which is especially important for short reps that are like 1 min or something), but also cardiac drift, etc
If you look for a training plan that's for where you are now with your running, it should give you a good idea for what paces should be on workouts, easy/long runs, etc. And I'd really recommend that. Those Garmin workout suggestions are always going to make not so much sense because it's all just based on a few numbers that don't represent reality well and are very specific to individuals, have a lot of caveats, and don't really tell the whole story. I suspect Garmin and other companies just release all this kinda features because it just sells well - "magic AI coach that analyses your data and give you suggestions". But the truth is that a lot of it just useless metrics and numbers, and having them displayed all the time in your face will often actually make your running worse.
Running's a very simple sport. If you take a proper training plan and just execute it, you're almost guaranteed to get better, be injury-free.
I think a lot of good advice here, but could also additionally recommend a mobility routine, especially around hips. Poor joint mobility, which unfortunately most of us nowadays suffer from, affects quite significantly how one moves and usually results in extra stress on the knees.