Victoria
There are training plans available for download on this website...
Look at the top tabs. (Home,Events, Training, Health Gear, Forums)
In the Training section there's another tab called "Racing" which contains a list of links to "key articles"
OK you can read them all, but obviously I would suggest reading the "racing basics" and "marathon" and "RW Schedules week 9". You will also find plans for "The get-you-round schedule" and "advanced".
It's always worth looking at the plan for the next grade up, to see what it is that they are supposed to be doing.
My humble suggestion is that your primary concentration (as indeed you intimate) is the single long endurance run. BUT more miles done means that this becomes easier. Try and get 30 to 35 miles per week somehow, but don't despair if you miss the odd slot.