Typical cross training for a runner includes yoga, strength and cardio workout.

Yoga increases mobility and flexibility, but for beginners some simple stretches before and/or after running may be enough.

Strength training can reduce the risk of injury. However, beginners are able to increase strength just by running, especially hill runs.

Cardio workout like cycling, swimming, rowing, 1b1 and elliptical can be taken as a substitute for running to improve endurance, when the impact on joints is too high or the weather is too bad. However, it is possible to avoid high impact on joints via building up mileage gradually with a lot of patience. As for bad weather, it is possible to run in an indoor sporting arena or on a treadmill instead.

So cross training might be optional if the goal is just to finish a marathon.