Most people believe that to become a better marathon runner or triathlete, a person must run long distances for long durations along with doing high-repetition training in the gym to build up endurance. To a certain extent, this is true, but research shows that endurance athletes who also do heavy strength training improve much faster.
In a systematic review, athletes who did strength training twice a week whilst maintaining their endurance training increased their running economy (ability to use less oxygen while running at a specific velocity), maximal strength, neuromuscular connections and maintained the same levels of VO2max (maximal oxygen consumption) compared to those who ONLY did endurance training. This meant that those who strength trained also improved their race-times.
Strength training’s improvements on running economy and velocity are because of improved leg muscle and tendon stiffness which allows the rapid absorption and use of elastic energy during every foot stride. The optimal amount is to do your heavy lifting twice a week and on non-consecutive training days. Therefore, to improve your marathons, don’t just run. Incorporate some heavy lifting and you will surely outperform your competition!
Beattie, K., Kenny, I. C., Lyons, M., & Carson, B. P. (2014). The effect of strength training on performance in endurance athletes. Sports Medicine,44(6), 845-865.
Millet, G. P., Jaouen, B., Borrani, F., & Candau, R. (2002). Effects of concurrent endurance and strength training on running economy and VO~2 kinetics. Medicine and Science in Sports and Exercise, 34(8), 1351-1359.