I think it's time that Garmin add a marathon training plan to the connect app sooner or later every runner wants to try the epic distance.
Please Garmin add it
As tmk2 wrote, check out the self-guided marathon training plans in GC Web at https://connect.garmin.com/modern/training-plan?type=Running

This is not available with my watch maybe? These are my options with Garmin Vivoactive 4S
Click on the link in my previous post, and start the Training Plan from there.