@mikesf90 first, when you're in doubt you go to see a doctor, not a community forum 🙂 It doesn't mean that something is wrong but simply it will put your mind at ease to know whether there is anything to be worried about.
Lower RHR is often expected with an increasing level of fitness. Even slightly overweight your RHR may go quite low because it's also driven by your genetics. I'm in fact one of the examples as my RHR was always oscillating around 60bpm despite being obese (to the point NHS calculator ran out of scale on the BMI gauge :)). After I started dieting and getting more active I noticed my RHR dropping more into the 50bpm-60bpm area and just in case I consulted the doctor (same worries as yours). My current resting HR is between 39bpm-42bpm (unless I detrain for some reason like during injury, then goes up) so as you can see, it can go very low and it can be absolutely fine.
The problem would be if your RHR is low and you find it difficult to elevate your heart rate during activities. From what you are saying, the problem is not there. As well as you have a lower RHR, you can also elevate your HR to higher levels. Considering your max HR (and I assume your watch doesn't lie, I switched to the chest strap, I don't trust wrist HR monitors) you have quite a wide HRR (Heart Rate Reserve). This, with lowering RHR, is usually an indicator of improving fitness level. You may find out your max HR by trying to go all out short (30sec-1min) intervals but for that, I recommend using a chest strap (and forget about 220-age, max HR depends on a lot more things including your genetics).
Saying Zone 5 I understand it's not Fitbit's 3-zone model. What model do you use for zones? Is the 5th zone considered "anaerobic" (is it the last one?)? If you do the whole run in zone 5 then your zones are set wrong and your Anaerobic Threshold (binds to the HR over which fatigue starts piling up quickly) is quite high. Basically, you'd not be able to run the whole run in an anaerobic zone because your muscles would eventually give up due to lactic acid build-up (normal thing, nothing to worry) 🙂
You say you feel dizzy. Are you currently on diet (in a calorie deficit)? Sometimes short dizziness may occur and there's nothing unusual about it (for example, quickly standing up from bed right after waking up and there's a chance you'll feel dizzy for a few seconds). Being low on carbs may cause that, too etc. There are many reasons and it may also include medical issues but that's for the doctor to tell you.
So, as I said in the first paragraph, if you have doubts then ignore what I said and just see the doctor so you'll get the answers you need 🙂
