Not built like a runner and therefore anything around 50 miles is a walk pace for me. Did 15:02 for a 54 mile event in September, I was joint 13th with only runners in front of me and a few runners behind me which we overhauled in the dark due to better constant pace and better navigation. Have done 6:15 for a hilly, off road thirty.
For a thirty I don't change much from marathon training and just use walks on uphills (not upflats) and some steep descents due to sanity.
However I have read some stuff on endurance training as I used to do national trails 20 miles per day multiple days with backpack and this stuff just interests me.
Training is about building the body up to be stronger and fitter not breaking it down. So if a 40 mile training run wipes you out for two weeks then it is bad training. You can afford to wreck yourself in the event itself if that is your only goal and that just requires mental toughness not physical ability.
Learning how to run on tired legs is important and two shorter runs with a rest between will be less detrimental to your body than one long run. e.g. two 20 miles rather than one 40. Building training volume is important, walking wise this is termed time on your feet and 5 lunch time walks of 3 miles (45 minutes) adds 15 miles to the weekly volume. Not stressful on the body and helps the body get used to time on your feet. Reduces chances of failure through blisters, knees and hips seizing up.
So you need to monitor your training as you build up. Write down your stress score for physical, mental and emotional factors such as hectic work, family, travel schedules, sickeness and especially the training schedule. Write down credit points, for sleep rest and theraoy, a good nights sleep, stress free life, massage anything that is a plus. It is subjective but an imbalance for too long will reduce the benefit of training. (Rick Crawford a cycling coach gets the credit for this not me). Also rate how hard you are finding the regular training sessions that you are used to e.g. the 6 mile evening circuit and be prepared to back off.
Easing off and putting in a good 30 mile effort will boost mental confidence for the goal, quick rest and back to training. Two 15 milers back to back may have the same training benefit but won't boost confidence as much.
One of things to consider is how long it takes to recover from a marathon. If you are back at it in a matter of days then you are able to cope with a higher volume of training and therefore the finish time for the 50 might be more important than just finishing it. Equally if it takes 3 weeks to recover then doing any runs over 26 miles in training would be counter productive.
I did a thirty mile event on a wet cold Jan day and commented at the finish that I hoped to be walking by Thursday, (on Sunday). The guy next to me said bloody hell he better be back to normal by Tuesday as he had a marathon on Ben Nevis (or some other stupid big hill) the following weekend!
Not a training plan as such but just stuff to be aware of around the training plan.
DG