One issue with riding a tandem long distances is that people tend to move around less on the tandem than a single bike, which leads to fatigue.
You can manage that by making a conscious effort to stand periodically, and change positions.
Ultimately it's all what you're used too. Our long ride on the tandem is 167 miles Cross Florida (one day).
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.