Mixed Intervals are just that. Once again, this is an opportunity to have a little bit of fun with your workout. One of my favourite workouts in college was The Mystery Workout. There were several different distances in a hat, for example, 8 distances for 8 athletes. Only the person who picked the interval out of the hat knew how far the interval was.
Furthermore, the example workout below is a phenomenal way to replicate a race. The glorified strides (200m) simulate the start, the 1000m allows you to find your rhythm, the hills spike the heart rate (similar to a race), the 1000m enables you to find that rhythm again, finishing with the 200m repeats for a fast finish!
When: Any phase
What: Intervals are various paces and distances
Where: A clear path
Why: To add variety to a session while stressing various energy systems. This is also a great race simulation workout for cross-country runners and/or road racers.