I started getting creative with different ways to link up trails to get rides like that. I ride North Park a lot and found this approach adds a little variety. Sometimes, you have to work in a little road ride to link up trails. Yell at some roadies on your way to feel better. Here are some options. I find that parking at the Spillway (or cannon), or the Boat House gives the most options for linking up trails and then ending up back at your car.
Park at Spillway or cannon, up the hill and drop in to the right, then shooting range trail OR Koto Buki trail, (possibly add street ride to Girl Scout climb and Front 9 trail), then street ride to Compost Pile / V8 trail, then Back 9 trail, then options by the Bathtub trail. If you feel good you can even then add the North Ridge trail.
Park at Spillway or cannon, up the hill and drop in to the right, when you get to West Ridge Road, head back over to do the Pool trails. If you do all the trail and some fire roads, you can get 8 miles up by the pool.
Park at Spillway or cannon, up the hill to the Pool trails, then Koto Buki or shooting range trail, then street ride to Front 9 trail or Compost/V8 trail and/or Back 9 trail.
Park at Boat House, street ride to dog park, up the steep hill on the road, then Compost/V8 trail, then Back 9 trail, then options by Bathtub trail, then North Ridge trail.
Park by Ice Rink, then North Ridge trail and Pfundstein extension, then at bottom of hill near the Spillway, go back up the hill on the trail and do the OLD North ridge trail. You can also park at the Spillway and do this in reverse.
Park at Boat House, street ride to North Ridge trail, then short trail behind townhouses, up to the Pool trails.
It's hard to explain. I could show you some loops.