Has anyone here run a marathon?
Not me.

At the moment I'm comfortable running about a half-marathon distance.
That's rather awesome. Go you!!

I need to increase that to a marathon distance.
But why? It's highly unlikely that a tiger/zombie/raptor would chase you for longer than 21km. So by the time a tiger/zombie/raptor *could* have caught you, it will be very full from all the people who cannot (or did not) run away as long and far as you.

Has anyone here trained for a marathon?
Probably - but not me (see above).

How long did it take you to increase your distance? Did you use a particular program/coach?
Haven't. Didn't. No. Mostly because I don't like running.

But yay for you, and good luck!

I have heard the zombie run app is quite good (seriously) but I think it's unlikely it would prove a challenge for you.

Good luck!