関連ワード:  how hard is it to train for a 5k    how hard is it to run a 5k    how hard is it to run a 5km    how hard is it to run a 5k without stopping    how hard is it to run 5k in 20 minutes    how hard is it to run 5k under 20 minutes    how hard is it to run a 5k without training    how hard is it to run a sub 20 5k    how to train for a 5k beginner    how long to train for a 5k beginner