This summer I "discovered" a hiking/biking combo along a local backcountry trail, and I like both parts. I park my car at one trailhead, bike on the road to the next trailhead, lock the bike, and hike the trail back to my car, then drive back and pick up my bike. The trail is for hiking only, so the bike part has to be on the road. Typically it's about 6 miles rugged hiking and 10 miles biking on gravel, both consisting of rolling and short/steep hills.

I feel like the biking segment is more aerobic, but both are pleasingly difficult.