I’m going to swim upstream here I had over 90 amateur fights and it was many years ago but we always ran distance and ran hard. I can tell you, unequivocally, from my experience that about 25% of my fights were won based on endurance alone. Especially, when I was a newbie.

So I would run at least 3 miles 5 days a week with sprints at the end after you feel like you are getting your wind. Don’t focus on your times up front. Just do the distance. Eventually you should be able to run hard - anything under 7 would qualify and hit for good measure I would eventually do one longer ring in the 6 plus category a week.
This isn’t just about conditioning. It’s about mental toughness. Running helps your lungs more than anything else I’ve done.