Hi Stefan
Pictures make things so much clearer! If it were me, I would clear an area behind the tree for running, and yes, I can run, at least for short distances. Then I would bind the tree as Paul describes. Finally, I would make a horizontal cut on the outside of the curve. As SOON as I heard wood starting to crack, I would be on the move away from the tree as quickly as possible. Let it settle and then come back and do it all again. The biggest thing to watch for is as the tree parts company with the stump, it will have a tendency to kick, or jump in an unpredictable direction. If you can leave little bits of wood connected (the hinge), they will buffer that jump and protect you from injury. If this is not clear, by all means ask for clarification before you cut. We would hate to see a notice about you being run through with a tree trunk or squashed like a bug.
Bill