It's really quite amazing how quickly it affects aerobic endurance as well. I had a knee injury, waited 9 months for surgery, recovered, did a self-planned physio routine, and started feeling ready to get back into aerobic exercise.

There was a 5km fun run in town that I figured I'd use as motivation. I did a slightly modified HIIT routine: walking warm up to the football field a few blocks away from my house, 8x flat out sprints from end to end with 15-30s recovery, and then cool down by walking back home.

I went from probably 15 months of completely sedentary life to running a 23min 5km race in about 6 weeks. I ended up finishing somewhere around 15th in the race, and every person who did better than me was listed as "M (18-23)"... and I don't even pretend that I can compete with testosterone-soaked young men :D