On all the trails I've been on so far, 30 miles round trip has been the longest... mostly because the trails around here are so rough that I only see maybe 20mph for 30 seconds the whole trip, haha, usually we're crawling over rocks and thru streams at 5-10mph. My wife and I were out today and only put 20 miles on them. Here's some video of what most of the trail consisted of (Seven Mile):

Seven Mile Trail