The only way i found that out, was to enter a challenge walk of a distance i thought was beyond my capabilities.
Up until my first South Wales Marathon walk, back in the 1980s, the furthest i had walked was around 32miles, over fairly easy terrain.
At the end of that challenge event, i was spent, my legs were sore, and i do not remember my head touching the pillow that night, i was beyond tired.
The South Wales Marathon walk, was close to 53-54miles, over the entire Brecon Beacons National Park, from one end to the other, and for someone who had never walked that distance before , i thought it would be a good learning curve, on what my body was capable of.
When your in the company of fellow experienced walkers, who are used to walking such distances, and the accommodation and food was laid on throughout the event, then it was just mind over matter.
I knew most of the terrain really well, so completing the event in just a little under 16hrs, proved my hill fitness was far better than i realised.
Years later, i discovered around 80miles was my maximum limit.
Those days have long gone, and i recon i would struggle to complete a 30miler today.