I’m finishing up the run maintenance level 0 plan running nearly all on trail. Its definitely improved my running economy and helped manage foot busitis. I dont have any race goals at this point, only have 5-6 hr of time each week, and while i do want to see if i can get the fitness do a big vert ultra a year or two from now, i want to focus on improving my aerobic threshold pace for a while without getting injured (its abysmal). Since im running rooty/rocky trail, im not too keen on speed intervals, I’d prefer hill intervals if that is a plan differentiator (or thoughts on swapping farlek runs with hill repeats?).
You can turn any interval session into a hill reps session simply by running it on hills. You don’t have to change the format at all. Just do it exactly as it’s drawn up, going uphill for the hard efforts and downhill on the recoveries.