It is not uncommon for multi-sport athletes to attempt completion of a marathon at season end. Typically, Triathletes are quite fit at this point of the season, but under trained on the run for a transition to marathon specific training with its more demanding run volume.
As you transition, ensure you keep a healthy mix of swimming, strength, and biking in the mix in the transition weeks. These additional workouts will keep your fitness up while you more gradually change the volume mix to more frequent running and ultimately longer runs. Don’t rush the transition. Trust in your fitness and focus on progressive adaptation to more running.