The Great Wall of China
The Great Wall of China is a well-known landmark, but what you may not know is that 99 percent of it is off-limits to tourists. Just 1 percent of the 4,000-mile-long (6,437 kilometers), centuries-old structure is open to visitors [source: de Bruxelles]. In 2006, an American couple and another duo from Great Britain gained permission from the Chinese government to embark on separate journeys to walk the length of the Great Wall.
If this is your dream, then it's never too early to start conditioning. You'll likely log an average of 30 miles (48 kilometers) a day for more than a year, hiking through varied terrain spanning stretches of mountain regions and marshes. What's more, much of the wall is completely eroded. In other areas, the wall is extremely narrow. Although it can be as wide as 30 feet (9 meters) at its base, the path across the top of the wall narrows to less than 12 feet (3.65 meters) in some spots. Then there's the issue of food and shelter. Sure you can carry your tent and sleeping bag, but who can carry a year's worth of food and water-sanitizing tablets? Even in top shape, you could get dehydrated, come down with a gastrointestinal disorder or have to take a few days to rest due to spine compression (as did one of the young adventurers who eventually completed the trek back in 2007).
But, of course, you don't have to travel to China to find a long, tough, trail.