Consistency is key, start small and be patient.
HIIT is a great way to improve VO2 max, I've seen significant gains from it myself. I'd also add that hill sprints can be super effective. Find a route with a steep hill and sprint up it at max effort for 30-90 seconds, then walk or jog back down to recover. Repeat for 3-5 sets. It's tough, but it'll get your heart rate up and push your aerobic limits.
I'm a big fan of active recovery too! I like to add some light yoga or Pilates to my routine to help with flexibility and strength. And yes, contrast showers are amazing for reducing inflammation. It's great that you're experimenting and finding what works for you.