Interval training alternates short bursts of fast, hard running with periods of easy running or walking to recover. It's used to build speed, running economy, and aerobic power in a structured, repeatable way. Most training plans schedule it once or twice a week, never on back-to-back days.
It's a workout structure where you repeat fast segments, like 400m or 3-minute efforts, with defined recovery jogs or walks in between. The hard and easy portions are usually set in advance by distance or time.
It depends on the goal, short intervals of 200 to 400 metres build speed, while longer ones of 800 metres to a mile build stamina at a hard, sustainable pace. Beginners typically start with shorter reps and fewer of them.
Once a week is enough for most runners, twice at most if you're experienced and recovering well. Doing intervals too often raises injury risk without adding much extra fitness.
Intervals follow a set structure, like 6 x 800m with a fixed recovery time. A fartlek is unstructured, you vary pace by feel during one continuous run.
Not right away. New runners get more benefit from simply building a consistent easy-paced base first, then adding structured intervals once that base is solid.