When is the marathon?
are you planing to only run until then, or could you fit in some cycling?
Start with doing 5 miles for 4 times, see how you feel, increase the distances by 5 miles till you can comfortably do 50/60.
Ride, if you have time, at least twice a week.
Then, on the day, you will manage 80 no problems.
You need to do some cycling prior of the event not only for stamina, but also to see if the bike fits, if the saddle is comfortable, to find out how much food and drink you need on the big ride, to learn how pick the right clothing.
For example, you will need gloves and eye wear on an 80 miler.
If you don't do the things above you will probably still make the 80, but you will suffer during and afterwards
