If you're a runner, then tempo runs, long runs, and speedwork will probably be part of your training routine, as they will help build speed and efficiency. Strength training should also be part of your training regime, and here's why:-
# prevents injury by strengthening muscles and connective tissues
# helps you run faster by boosting neuromuscular co-ordination and power
# improves running economy by encouraging co-ordination and stride efficiency
Still, many runners will not incorporate strength training into their regime for the fear of gaining weight and slowing them down. However, you would need to lift regularly, and be eating a large amount of extra calories for this to impact on performance. The focus should be on gaining strength and power, rather than bringing weights into a metabolic workout. Weight should be heavy, for 3 sets of 10 repetitions, with full recovery. As confidence in lifting improves, then weight can increase and reps can reduce, and sets can increase. Compound movements such as squats, lunges, rows and deadlifts will give you maximal gains, rather than isolation exercises.
Good lifting form is crucial in preventing injury, starting with body weight exercises, and then slowly adding weight. You can increase the weight every two weeks, making sure you are being sufficiently challenged by the last few repetitions of the third set. Lifting once or twice a week would be ideal, integrating intense runs on days that won't be impacted by preceding strength training sessions.