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 zone 1 - zone 2 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!