For optimum gain you should mix moderate exercise with intense exercise, but the time you put in likely made your later success possible.
I did this on purpose this spring.
I had a shitty winter in terms of endurance and was pretty much starting from the gutter in February. Spring and early summer we're stressful so I did mostly Z1/LSD cycling in the form of extended commuting from work to home. Nothing excessive though as stress was plentiful.
The estimated fitness or the Garmin estimate didn't budge much.
Once vacation started I kept this on for few weeks to recover and after that added few harder sessions in a two week period. Few weeks after this I felt the change and Garmin also estimated 6 mil's of added VO2MAX in a week.
Don't care about the metrics, but the added QOL from this and now I hopefully have a structure for the coming winter not to lose as much of this as the previous.
This is about health.
High volume of exercise is damaging via oxidative stress and other ways. Weekly training volume is the most reliable predictor for injuries.
If health and basic fitness is the goal, rather than bringing home the gold, you can add intensity to just a fairly small volume of moderate exercise, or perhaps even none at all, other than a warmup and cooldown.
Then once the base is built, to make any progress one needs stronger, more focused stimulus, so you introduce interval training at high aerobic (tempo and threshold paces) as well as anaerobic (400-800 "very hard" sustained and all-out sprints) training, combined with a few zone 2 long runs.
And then the cycle repeats: you have to let the body recover so deload, then start a new cycle.
Nicklas Rossner has fantastic resources; there's a video there that explains how to find your current bottleneck: https://m.youtube.com/@NicklasRossnerPT
I've messed with zone 2 a bit and read a fair amount about it. If it gets you out there moving consistently: great. The best workout plan is the one you do. But otherwise I really think it's more of an optimization strategy for people who are trying to seriously increase their miles per week, and are pairing it with higher-intensity runs and/or weight training.
To be clear, performance-maxxing was never the objective. I am in it for the health benefits and conditioning for grueling pursuits. Nonetheless, observable effects of different kinds of training on performance are still interesting to me and helps me optimize the time I spend on it.
There are plenty of online training program generators that are pretty good and will take you from zero to running 10k. I used an online program to run my first 10k somewhere circa 1990.
There a lot of more cutting edge science online (and I follow Nick as well) but if you're not a competitive athlete and are just trying to get in better shape then the answer is mostly just do it. Over some base of running you can throw in some interval training including HIIT.
After that it's probably determining your LTHR accurately (don't just accept what your Garmin or Apple Watch says it is). This gives a better sense of how hard to push on your hard days, and it's also useful to see what % of HRmax your threshold lands at.
It is also possible you were over-training in your "moderate" intensity or conversely not increasing the load/distance enough. Training over years should result in good improvement even without HIIT. You will need to put a lot more time in.
To be clear, I was objectively in good cardiovascular condition before adding HIIT. My resting heart rate has been in the 40s for years. The surprise was that there was so much easy margin left by adding HIIT to my regimen.
It’s always that lady who chilled all day who says laughter is how she lived so long!