If you've spent time poking around the FlyerTalk forums or you've read frequent flyer blogs, you may have seen the term "mileage run" pop up from time to time. No, this isn't an opportunity for travelers to sneak in some physical activity on a long trip (unless you count the walk from gate to gate). Instead, a mileage run is a very low-cost ticket that a frequent flyer books with the sole intention of earning miles, or reaching the next elite status level at the end of a calendar year.
When airlines publish super-cheap fares by mistake or discount their tickets between unpopular city pairs, frequent flyers have an opportunity to earn a tremendous number of miles without spending much cash. These rates are usually lower than what an airline charges for purchased miles, and they count towards elite status, too. If it all sounds crazy, that's because it is. Many travelers book trips that don't even have them leaving an airport, with the most extreme flyers opting for multi-day international itineraries that wouldn't be pleasant to fly even if they did have plans at their destination.
Other, more rational travelers book extra trips at the end of the year, taking a vacation that they wouldn't have otherwise in order to reach the next airline or hotel elite status level. If you're Gold and you only need a few thousand miles or another couple nights to reach Platinum status, for example, it could make sense to book another trip to reach that level, especially if you anticipate a lot of travel in the next calendar year. These trips may or may not be to destinations you or your family want to visit (weekend fares to Alaska are cheap in February because leisure travelers wouldn't normally want to travel there), but if you're flying somewhere you've never been, it could be worth it to go on a mileage run of your own.
More often, though, mileage runners are traveling solo and in a single weekend day, so they can save vacation days for actual vacations. A popular routing is New York to San Francisco or Los Angeles, where flights can be very competitively priced, especially when one airline is having a sale (and competitors opt to match the low fare). Passengers can rack up a lot of miles in a single day, and can fly back and forth without taking a redeye (although a night on a plane isn't unheard of during a mileage run).
Elite members may even score a free upgrade, making the experience a bit more pleasant, especially if you have some reading or movies to catch up on.
When speaking of mileage runs, frequent flyers usually calculate the cost of each flown mile. Say you're paying $250 for a cross-country ticket that would earn you 5,000 miles roundtrip. That booking would cost you five cents per mile, making it a decent deal, especially if you're earning a bonus. If a frequent flyer is traveling just to earn redeemable miles, though, with no plans to earn elite status at the end of the year, the rate would need to be much lower to make sense as a mileage run. Twitter is a great resource when researching deals, with sites like TheFlightDeal often publishing discounted fares many times every day.