I would gladly train for marathon, not due to midlife crisis or to show off but to prove myself that I'm capable of doing it.
Unfortunately my pb is 10k and my body told me that's enough. I have to listen to my body and not work against it.
Doing two trainings a day is not healthy as you're overtraining yourself and it's going to have consequences.
I'm planning to start running again a couple of times a week but first I need to take care of some health issues.