alt Hacker NewsHIIT can have a pretty quick impact on your VO2MAX but that is going to level off. As others have said you will want a more balanced routine in the long run.
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.
I have a pretty diverse training load these days. Hiking, mountaineering, running, plus a few sports and some resistance. While I have overtrained in the past on running in particular, it was mostly due to lack of diversity. Nonetheless, none of those things puts the same load on the body as HIIT.
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.