Don't do HILT until you have built up a long training history of running, it's not for beginners. Also make sure you do such exercises sparingly, about 10-20% of your weekly running volume, otherwise you increase injury risk dramatically. Accelerate into each interval slowly, and decelerate slowly afterwards, no sudden changes. Take an easier training day afterwards and a complete break from HiLt every 4th week to facilitate recovery. Then el both of the interval doesn't matter too much, longer intervals are safer and prepare you slightly better for longer running distances, 800m is my favorite but I'll do 400m and 1600m as well. The longer intervals build lactate threshold tolerance. The shorter distances are better for leg strength and form but have a higher injury risk.
A running track is good because you have a softer surface, treadmill also works as you get some springiness and can set a moderate incline which reduces impact forces. Otherwise as above, hill repast are very safe and build great strength.