Gaining fitness, or endurance, as a runner is fairly simple: Stress your body appropriately, allow it to recover from the stress, and an adaptation will occur.
But many athletes run too hard, causing too much stress, or they fail to recover properly, which doesn't allow the body to adapt fully. Before we go into what workouts will improve your fitness/endurance, you need to be mindful of the relationship between the appropriate stimulus and adequate recovery.
Recovery doesn't have to mean taking a day off or cross-training the day after a hard workout or long run. An easy recovery run does a lot of good the day following a workout or long run because it gets oxygen-rich blood to damaged muscle tissue, which will help you recover faster.
The following eight workouts can help improve your endurance for races from 5K to the marathon. Threshold and long runs are excluded from this list because there are so many articles and examples of those workouts, but I consider those two workouts vital to improving aerobic fitness, or endurance.
Fartlek is a Swedish term that means "speed play." Fartlek workouts help the athlete learn how to feel different paces, and this skill is important if you want to run at your fitness level in a race, rather than going out too hard, then slowing down as the race progresses. Learning what pace you can sustain for a given distance is important for consistency.
The first part of the fartlek is the "on" portion and here you want to be running a challenging, but controlled pace. The second part is "steady" and this is the key to the fartlek workout—you're not running slow and you're not running your easy pace, but rather a pace that is faster than your normal easy-day pace, but obviously slower than the "on" portion. You never get a full recovery with a fartlek workout because the slowest running you're doing is steady running.