Afaik you can only ride on state or federal owned land
For the most part, that is largely true, but there is so god-awful much of it available, especially here in the west. I have 40 miles of trails that start 150 yards from my front door and another 150 miles within 20-30 minutes or so--ranging from 6000 ft to 10,000+ ft. What I really love the most is that I can go on most of the trails statewide and, often times, except for some close-in trails on weekends, not see another person: hiker, biker or horse-rider.
Of course, it's not that way in many states-what they call mountain biking trails we call foothills
On our next trip to the UK (hopefully this summer or next and I don't get too damn old to ride), I am going to have to make a point of renting a bike and exploring the offerings.