Er, I think the motorbike Enduro people were there first, and the mountainbike discipline is pretty much the same format. Some people in the UK started calling some marathon events Enduro as explained above, but it has reverted to the proper usage now and fair dos to them for sticking with it ! But it must cause confusion and some significant over-biking at their events. All good fun.
If you believe Wikipedia, the first motorbike Enduro was when the International 6 Day event took the name in 1980.
So in answer to your question, it started off as riding timed sections on an offroad motorbike in 1980, was use to describe marathon mtb events briefly in the UK only while the rest of Europe used it in the sense it has now become accepted worldwide, riding timed sections on an offroad bicyle. That usage has now been universally adopted.