In reply to Robyn Vacher:
I would personally be very careful about using some of the pre written plans available on the internet.
I'm doing Edinburgh next May as well and a lot of the plans suggested 5 days running.
Now I'm not the same age as Lesley, I'm 37, but will be running 3 days a week to ensure I can still do my circuit training once a week, climbing and mountain biking.
I've had a plan written for me which slowly builds up the miles. I am prone to injury as I tend to push it too much too soon. So my plan gives one long run a week, sometimes off road, sometimes on road.
My two shorter runs vary each week. So one week I'll be doing hilly runs and the other interval sessions. One of my shorter runs will be at a very easy pace.
I'm aiming to do under 4 hours.
I have two friends who had plans like this written for them prior to Berlin Marathon this year and they managed 3 hours 42 and 3 hours 47. They both ran three times a week and remined uninjured.