Easiest thing to do would likely be join a local running group that has various training distances/pace groups
The one I ran with had two annual sessions for half and full training (a long with 5k/10k ).
When I was training for my first ultra (Yamacraw 50k) I just did the full plan with the training group + 4 miles extra on the long Saturday run. Typically I could find a fool to run them with me :)
At least some of Garmin watches have workout suggestions for daily motivation as well as actual training plans you can do. (Found in the connect app) I don't think the suggestions are always great but what you can do is see your training loads in the different zones and decide if you want/should do some intervals or not. My experience with suggested workouts is I can get multiple days of high intensity in a row which I know makes no sense but the watch is trying to get time in that zone in the "range"