Sounds like you are doing it right.. As long as you are truly sprinting -- as fast as you can go on the straight portion and walking to recoup. I personally wouldn't be able to pick back up and run the very next straight section at a full blown sprint, but I haven't been running like you have for the last 4 months either.
Push yourself to your limit and then beyond on your sprints, and then recoup and it seems like you have it.
When I did
HIIT, I was sprinting at 12mph on the treadmill for 15-20 seconds and then walking for like 1.5 mins to cool down before I began again.
Good Luck.