For 5k training the amount of reps can be anything from 3 to 30, but should not really be any longer than 1mile in distance. The reps can be run straight through or broken up into sets with a longer rest period between each set.
The benefits of using rep sessions are that the athlete’s body will adapt to cope with lactic build up, as well as increase its ability to deliver and use oxygen. Reps can also reduce the risk of repetitive injuries associated with longer continuous running, as well as the typical running benefits such as weight loss.
These types of sessions should ideally make up at least two of your training sessions every three weeks and be around 10 - 20% of your total amount of training.
Below are 2 sessions used by a previous Amateur Athletic Association 5000m winner:
20 x 400m with 60s recovery between each (which can be split into 4 sets of 5 400m).
8 x 800m with 90s recovery between each (which can be split into 2 sets of 4 800m).