Lacing up for a 5k marathon? In case you are not a regular runner it will be difficult for you to complete the race and you may injure yourself during the marathon. You need to follow a 4-week fitness routine to improve your running and ace the long-distance races. Nisha Varma an American College of Sports Medicine certified exercise physiologist shares the week 1 fitness routine for 5k marathoners. The focus for the week is endurance.