I am currently in the process of training for my first 50K. As part of that training, I ran the full marathon in early June at the Yankee Springs Trail run for part of my training buildup. Technically, I wasn't "scheduled" to hit a 26 mile training run yet, but I really wanted to do it this year since I ran the half there last year and really enjoyed it. I planned to run the marathon slowly and treat it as a training run, because I wanted to make sure I could keep on with my normal 50K training volume and not need to take a ton of time off after the race.
Short & Brutal: this may be the toughest 5K in Michigan. Robinette trails and brutal Provin Trails full of sand dunes, corn fields and tons of off-trail scrambling and hills. You will never forget this trail.
Long & Challenging: the famous Robinette trails coupled with the lightning fast Kuyper College trail. You are running two-thirds of a mile longer so you need to blaze. 3.65 miles.
All on dirt. 95% singletrack, but with plenty of passing opportunities.