Trail/Road Running Training

Hey guys,
Not a backpacking question but rather running. Over the years to train for backpacking I have done a fair share of running and have grown to love it just as much as UL backpacking. So here's my question for all of you seasoned runners. If you were to do a 30K road race, how far would you make your longest training run in prep? For the past 3 years I have run 3x week averaging 3-6 miles per run with some kind of hike or longer run on the weekend. My main motivation in running has pretty much always been for pure enjoyment. That said I enjoy pushing myself so I would not consider myself a casual jogger (i.e. my wife cannot understand why I love sprinting up the hills). I also try to switch up my training locations for each run so I am getting a good mix of technical, fast and endurance runs with 20-30 minute treadmill "hill workouts" on the days in between. In the past I have done several races on a whim with distances up to 20 miles and used different training plans for each event. So I am just curious how other would approach something like this. Thank you for your feedback.