Not sure exactly what you are asking as our definitions may not exactly jive but there are few really long mtn bike trails around unless you are at Moab or Henry Coe or some place in the black hills.
In the past I have done multiple loops of Long Ridge and In Marine you can without too much backtracking string together all the Headlands trails but truthfully most of the long trails that are worth riding have sections that are off limits to bikes. So if you tell anyone that my favorite ride is the full Skyline to the sea trail I will just deny it.