So I've been pushing up the distances lately. I ran 7.5 last weekend and when I got to the end, I felt I could have gone on. I designed a 10K route that would allow me to bail after 8 if I needed. Turns out I didn't.
What actually happened was that after adding 200 metres from a wrong turn, the cemetery I ran through had welded the exit gate shut. I totaled 11k in the end and again could have gone further. Might try running 15 next time and see how that goes...