The rest period for each type of speed training should be enough to drop the heart rate, but not to fully recover, so adapt my recommendations accordingly. So, if you did your 800's in 3:30 then its 1:15 mins break before you start the next rep. The break in-between each interval is half your interval time. You should be getting 1:30 minutes rest between each rep. Also, make sure you warm up before each session, and include a warm down phase. If you are new to speed training, then pick one you feel comfortable with, and use shorter repetitions at lower intensity, and with longer breaks. Obviously, you may need to adapt them, depending on your current fitness level and experience. My table below gives a number of different examples of speed training. If you have not done speed training before then I highly recommend that you start. Of course each type of session may include variations. One will be pure speed and another fartlek or tempo. Most club level runners will include 1 or 2 speed sessions a week. So if you are joining our Trail Running Holiday adding speed training to your programme will really help you get the most out of the week. It is also good for aerobic, threshold, endurance, and strength training. However, speed training, be it interval training or speed sessions, delivers more than just an increase in pace. As such speed training is an essential part of any runners training programme. As a running coach I can tell you that speed is essential in all types of running, from road to trail and even in long distance races.