8 Laps would put you at a little over 33 miles if your using the standard 4.167 mi laps. With a marathon under your belt I’m sure you would be able to hit your goal with a similar level of training following a 50k plan.
I’m assuming the 8+ lap goal is because you don’t plan on running all night, because of that, sleep (what most people consider the biggest challenge) won’t be a concern.
So not having to worry about a sleep plan, the biggest differences are going to be
- Total time on your feet
- Running on trail (assuming your marathon was road, if not disregard)
- Refueling throughout the race
From a training standpoint I would make sure you’re getting trail miles that are going to be similarly hilly to the race course, target that L1 L2 zone while training, and break up some of your longer runs into multiple short ones with 15-20 min breaks between them. If you can take a 12 mile run and break it into three 4 mile runs with short breaks you can dial in your refueling and recovery strategy and see what works for you.
Backyard ultra is a great goal and tons of fun!