I'm curious how those not involved in a formal running team or club go about training. Is there a specific app or program you use? How do you come up with and follow through on training plans?

I ran cross-country in high school. In my mid-thirties I picked running back up doing a C25K, then just kind of building endurance by increasing the distance. Eventually I found the Nike Run Club app and did it's half marathon training program, which I really enjoyed (I've actually done it three times now). Every week they give you two speed runs, two recovery runs, and a long run to do. All the runs are guided, which is nice having a voice in your ear telling you to run, stop/rest, how to change your effort, etc on speed runs.

Long term I'd like to run a marathon at least once. My other probably somewhat unrealistic goal is to run a sub 20 minute 5k, since I never could crack that barrier I'm high school. But I don't have a coach like I did back then, and Nike's programs are limited to the half and full marathon, so I'm at a bit of a loss of figuring out how to train.

Any thoughts, experiences, recommendations welcome!