Nothing wrong with mixing and matching as long as you understand what's behind the schedule and also what effect you are trying to produce on your body.<br><br>
If you are trying to work on VO2 max then you should be running at pace near your 15 min all out pace. For faster runners that's about 5k pace, but for slower runners it's closer to 4k or maybe even 3k pace. Intervals like this can be anywhere between a minute and 5 minutes long, but I prefer them around 4 min, which is approximately 800 meters for me. Should be the same for you too, based on your HM goal time. Recovery time can be about 2/3 to 9/10 of the interval time, but the important thing is to be fully recovered before starting the next one. I like to do 800 meters at interval pace (about 3:50 for me) followed by 400 meters at slow jog pace (about 3 min) on a track. It works well.<br><br>
But if you want to work on threshold then you should be running intervals around your hour race pace. For faster runners that's coser to half marathon, but for slower runners like us it's a bit slower than 10k pace. Slower intervals such as threshold paced intervals can be longer, (up to a couple of miles) and can have shorter recovery in between.