Some training ideas I use at Indiana Dunes State Park in Chesterton:
+ Park in the West Parking lot (this is the one West of the pavillion), run sprints from the parking lot down to the water and back up again, rest 1 minute and repeat. On the way down you get overspeed training and grade is low enough that you can maintain correct running form and you get gassed sprinting back up toward the parking lot. This is a killer and doesn't take long to get a good workout in.
+Just west of here along the beach is a good sized dune. Run (which will become a crawl at the end) up and down as many times as you can. Again, doesn't take long for legs to stop working and breakfast to start shooting out of your mouth!
+This same parking lot has a large wooden staircase on its westside, 20 bodyweight squats at bottom 10 pushups at top repeat.
+There is a newer (to my knowledge) trail at the top of these steps, I think it is trail number 3, that runs along side the fence on the far western border of the park at one point , this is a good sandy trail with a lot of mid sized hills.
+On the Northeast side of the Pavillion Parking lot there is Devil's Slide, good luck this is rough.
+Finally, there is a set of stairs along trail #8 that is insane.
+As mentioned above, any of the trails are good, however it has been my experience that deer and horse flies can be an annoyance in the summertime.
Hope this is helpful.