If you can run 1 mile you can bike 5. But if you can bike 20 miles it doesn't mean you can run 4 miles. Which leads to the paradox that cycling is more difficult.
When you run to your near maximum limit in competition you may be reduced to walking. However, do the same effort in a cycling competition and you just keep going, suffering. It's as though you hurt enough to quit but just enough to not quit. The pain is ALMOST insufferable...but not. And somehow (nature's way) in running you are forced to stop thereby ending the pain.
I believe nature's way, you stop running when it becomes intolerable. However that same type of intolerable pain when cycling is clouded/confused between man and machine and allows you to continue the SUFFERING!!!