Even if you are not training for an event, and therefore looking specifically to improve running performance, it can still be a good idea to include variety in your running. This can help with motivation, for example the increased focus needed for intervals helps disrupt the routine feel of regular easy runs, and will also be beneficial for your overall running fitness. Here are four categories of training runs and a few general examples of how those would be implemented as a workout.
Easy / Recovery
What? Very low intensity and short duration. 4/10 effort. Active recovery following a long or hard workout or a 'shakeout' the day before a race following a taper to help wake up the legs.
Why?: Helps the 'loosen' the legs after a hard workout or in preparation for a race. Increased blood flow (compared to full rest) gets oxygen and energy flowing to muscles. Mentally, as well as physically, less demanding.
How?: No more than 45-50 mins (above that starts to fall into endurance). Very easy pace perhaps combining walking and running. Able to have a chatty conversation while moving.
Long / Endurance
What? Low intensity and long duration. 5/10 effort. Might be prescribed as time (hrs) rather than distance especially when training for trail/ultra events where uneven terrain and hills will affect pace.
Why?: Increase muscular endurance and aerobic capacity. Mental training for the experience of sustained exertion. Taking marathon training for example, if your longest run in training is two hours but your target pace puts you in the 4-5hr finish time zone the second half of that race is likely to feel incredibly difficult even with an appropriate level of fitness . Practicing hydration and nutrition so that you know in advance of race whether your gastrointestinal system will tolerate your planned fuel.
How?: Slow conversational pace walking if needed. Planning a route can help avoid the temptation to cut short! To progress distances increase your longest run by 1 mile (or 10 mins) per week or 2 miles if you are already over 10 miles. Make sure to reduce volume every 4th/5th week to give the body a break during which time it can adapt to the increasing demands.
Threshold
What?: Moderate to higher intensity and longer duration. 7/10 effort
Why?: Increase lactate threshold (LT) - the level at which a hard effort can be sustained for a longer time but before a max effort zone where performance would begin to decline rapidly. Typically the pace that can be sustained for around an hour but no longer.
How?: Tempo - at or near LT pace for between 5-20 mins (plus warm/cool) this might included within a longer endurance run perhaps for multiple repetitions. Time trial - anything up to 10 miles, set time goal, at LT pace. Progression - escalating pace, perhaps one mile at a time, to set levels throughout the workout in a sustained way.
Intervals
What?: High intensity and short duration. 8-9/10 effort. Work and recovery periods are typically structured by time intervals (30 seconds - not more than 5 mins) but can also be distance (400m - 1 mile).
Why?: Increase maximum aerobic capacity i.e. the maximum amount of oxygen the cardiovascular system can circulate. May also stimulate muscular adaptation strengthening fast twitch muscle fibers.
How?: Straight - same duration intervals throughout workout (e.g. 3 mins work/recovery). Ascending - work intervals increase (30s/60s/90s). Descending - work intervals decrease. Pyramid - efforts increase then decrease.
A structured training plan is likely to include most or all of these types of training at some point during the process. Each workout has a purpose and a place within that training plan in order to elicit a specific adaptation to progress an athlete towards their goals. When and how you would use a workout will depend on individual goals (e.g. 5km or marathon) and existing fitness levels. Even when there is not a specific race goal in mind these can be used to enhance fitness and motivation.