Check out Hal higdon training. Pretty straight forward and effective.I turn 50 in Feb and I've committed myself to completing a marathon in 2020. It's a "bucket list" item. I'm placing no time requirement on myself - just to finish.
Worked up to 12 mi. so far, over 2 months of training. Most miles/week to date has been 22.
I have no idea what I'm doing, so I downloaded a "beginners" training plan and try to follow it religiously. That schedule is posted on my refridgerator, and being able to circle the miles after I complete them is a great motivator that puts me out on the road even when I don't want to go. The guilt associated with seeing uncircled miles is pretty good motivation too. LOL
Even though I played soccer for 13 years, I've never been a distance runner. I don't know anyone who is 6'4" and 210 lbs. who is a distance runner. Before this effort, the longest I had ever run in one shot was 8 miles, and that was one time. Prior to that, I considered 5-6 miles to be a "long" run.
Now that I've done 8-9 miles routinely, and 10-12 miles several times, running 5-6 miles actually seems pretty simple.
Please understand I'm running SLOW enough that I hesitate to even call it "running" but I figure if I'm not walking, I'm running. ha, ha.
Anyway, that's my current personal goal. I'm not allowing myself 1 second to wonder or worry whether I'll achieve it. There is no point in doing that. I'm just grinding through the process for the value of the grind, and the satisfaction of marking those miles off my schedule. If I never complete a marathon, it will still all be worth it to me. I'll have no regrets because I know I'm trying my best. I will admit, it is one of the most physically demanding things I've ever done since I don't consider myself a natural runner.