Endurance training can be divided into three areas:Basic, speed and anaerobic endurance.
Basic endurance training is for improving aerobic capacity and impact tolerance. Such runs occur at 60-75% maximal heart rate, depending on the runner's experience and level. At least one long basic endurance run should be included in your weekly schedule. They can also be shorter in duration. Such recovery runs last typically 30-45 minutes. Training should increase gradually throughout the basic endurance period (base 1 and base 2). At the transitional phase, general endurance training is reduced and once your main goal is in sight, reduce training even further. During taper time, basic endurance sessions are typically for recovery only.
Speed sessions are slightly faster than basic endurance training, and can either be even-paced or intervals. Heart rate levels during speed training should be around 75-85% maximum heart rate. Even-paced sessions last typically 20-60 minutes, while intervals are between 4 and 15 minutes (e.g. 4x8min/5min recovery, or in kilometers 4x2km/1km recovery). During speed training, breathing is accelerated, but only during anaerobic endurance training does breathing rhythm peak. Developing speed is important when training for a marathon, for example, since part of the marathon is actually run at speed training pace. Include 1 or 2 speed sessions in your weekly schedule.
Interval training is a good choice when you first start working on speed, since it's easier to keep up a good pace during short repeats and exertion levels are not too high. As you progress, you can add even-paced runs to your schedule. Long, even-paced intervals are recommended for marathon runners (e.g. 3x20min at 80% maximum heart rate, alternate with 10 min light jog at 60%). Cut back on speed training during transition and tapering, when you replace some of the hard sessions with actual racing.
Anaerobic endurance training is generally very hard interval work, aimed at maximizing racing performance and oxygen uptake capacity. To make sure lactic acid levels remain at a manageable level, run at just below full speed, in other words at 90-95% maximum heart rate. It's advisable to run at the higher end only during the last repeat. Aim to increase speed with every repeat. A session can comprise 5x3min with 5min recovery jog.
For a goal-oriented active runner, including anaerobic endurance training 2-3 times a month is advisable. Training should begin at base 2 and intensify gradually through transition and tapering. Some runs should be even-paced. When tapering, training includes anaerobic endurance and speed work, as well as basic endurance and recovery.
For an active runner, sprint training is fast-paced interval training at 90-100% maximum heart rate. Repeats last 45-90 seconds. Recover for around 5 minutes. Example: 8x60seconds 95% maximum heart rate + 5min recovery walk. Do sprint work during transition and tapering periods. Training frequency is at about 2 times/month.