Take a triathlon and stretch it out over a week or so. Add a few more sports, perhaps horseback riding or rappelling or ice climbing or kayaking. Or all of them.
Set the course in the most user-hostile terrain you can find, preferably in a country with plenty of parasites. Give the athletes only the sketchiest details of where they're going and make them navigate with wits dulled by pain and fatigue.
Now for the fun part. Make it a team sport and disqualify the whole group if even a single member drops out. Struggling together will solder the teamor shatter it.
Give the event a distinctive titleRaid Gauloises, Eco-Challenge, Southern Traverse, to name a few of the classicsand you have yourself an adventure race.