The way I see it is that I'm on a reasonably sized block of land where the boundaries aren't being used for anything other than attempting to keep animals in. So why not use em for getting fit on me mountain bike?. I have about four paddocks at my disposal at a total size of about ten acres without hindrance of gates needing to be opened and closed.
Slope = top to bottom @ about 1 meter per 10 meters. So not that bad.
So my question is, do I need jumps?.