Well. I'm toward the higher end of middle age, am in the worst shape of my life, have a terrible case of plantar fasciitis, and have never run a marathon. But I've always wanted to.
I have over a year to get ready and I think I can do this.
Just a piece of unsolicited advice, take the time NOW to get treatment for the plantar fasciitis! Do it right and don't skimp on it so that you can go into your eventual training strong.
My story - for as long as I had been running, I would get a stabbing pain in my calf when I would try to start off fast. After I did a couple of miles, I could pick up the pace no problem, but until then I had to go reaaaally slow. Stopping and stretching my calf helped a bit, but never for long. I looked for suggestions, but nothing seemed to work - warming up more, stretching more, strengthening my calves, changing shoes, you name it. Finally, about three months before last year's Princess weekend, I broke down and went to see a physical therapist who is also a runner. At initial assessment he diagnosed the problem (lack of ankle flexibility) and gave me a plan to get me running pain-free. A LOT of my Princess training was done walking since my running was strictly limited by my PT, but during the race, I felt 500x better than my previously races. My only regret was that I didn't seek out help sooner. (P.S. I also had to pay out of pocket for the treatment since it wasn't technically due to an injury, but again, it was totally worth it).
After my PT sessions were over, my therapist referred me to his training program, RunSmart Online, and it focused A LOT on strength programs for runners to prevent injuries. His theory is that injuries are caused by muscle imbalances and to tackle the root cause of the problem rather than the symptoms (i.e. the pain) and that the sooner you acknowledge the problem, the quicker it will be solve it. It makes a ton of sense once you get into it. Also, since he is both a physical therapist and a runner, he is very knowledgeable in both perspectives. (P.S. If anyone wants more information about the RunSmart program, let me know. I find it 100% worth it).
Right now, since there is almost nothing going on race wise and you have plenty of time before 2022, it is the perfect time to get in some strength and hopefully be ready for some injury-free training next year. Again, just my two cents! Take them for what they are worth...
