Boyoun Kim / The New York TimesAs they focus on fliers who produce the most revenue, airlines are making it difficult for frequent fliers to accumulate miles cheaply at year-end.

It’s the end of the year, and some posts on air-travel forums sound desperate. “Need 1,500 miles this weekend,” says one notice on FlyerTalk.com. “I need 1,820 miles to make Platinum next year,” proclaims another on Milepoint.com.

Implicit in such pleas, for suggestions for so-called mileage runs, is that the required miles must come cheaply — no easy feat when airfares have been rising and most flights are full, meaning choices are slim. Flying a specific number of miles as inexpensively as possible is the basic idea of the mileage run, that venerable institution of the airline frequent-flier realm.

And now the mileage run, at least as those in the cheap seats have known it for many years, is starting to fade away.

A mileage run is a trip undertaken solely to accumulate miles, usually to meet a goal of flying enough in a year to hit the mark for various levels of frequent-flier status — from 25,000 annual flown miles for the lowest level to 100,000 or 125,000 miles for the highest on major airlines. At this time of the year, fliers who need to top off their annual accounts with a specific number of miles often make one last lunge. Usually, they don’t care where they go, so long as they hit the mark cheaply by the time that ball drops on New Year’s Eve.

This year, in their drive to focus more on customers who produce the most revenue, two airlines, United and Delta, added a requirement for hitting elite status. Besides flying the requisite minimum number of miles, those seeking elite status for 2015 also have to spend a minimum amount with the airline — from $2,500 for the lowest status on United and Delta up to $12,500 for the highest on Delta.

Next year, those minimums are raised: $3,000 for the low ranks; $15,000 for Delta’s highest level. The requirements are waived for those who charge more than $25,000 annually on United credit cards co-branded with Chase and Delta’s American Express branded cards. Branded credit cards are a major source of revenue for big airlines. Delta says it earns $2 billion a year from its credit card and other relationships with American Express, for example.

There’s another twist ahead. In 2015, those two airlines will devalue their “award” miles for the lowest fares. For example, on a cheap $500 Delta round-trip ticket from Tucson, Arizona, to New York, I would receive 2,500 award miles in 2015 rather than 4,264 miles (the actual round-trip flying distance) for that ticket this year. In general, those paying higher fares, especially in first class and business class, will find it much easier than cheap-ticket fliers to rack up miles in 2015.

Besides those common miles, business travelers and others in the elite-status ranks use a currency called EQMs, elite qualifying miles, the only miles that count for elite status. “You get EQMs based on the actual flown mileage plus a multiple that reflects your elite status” and the amount you paid for the ticket, said Tim Winship, the publisher of FrequentFlier.com

“Mileage runs are largely misunderstood by people who don’t do them,” said Jonathan Spira, the editorial director of Frequent Business Traveler, an online magazine. Spira said he did a “convoluted” mileage run last year. The trip was New York to Los Angeles to London to Minneapolis to Los Angeles to New York, with the Los Angeles-London portions flown on a Delta business-class sale fare.

Mileage runs can be complex and far-flung, as canny fliers pore over complicated airline schedules and fare sales on various routes. One itinerary that was recently proudly described on an online travel forum: Dallas-New York-Chicago-Tucson-Dallas-Miami-Dallas-London-Dallas.

I’ve played this game, too. In late 2005, I commissioned a mileage-run expert to devise for me a three-day run from New York to Tokyo with two stops in Guam and Houston — 21,000 elite-qualifying miles, for a fare of $703.

American Airlines, busy ironing out its merger with US Airways, hasn’t matched its competitors’ mileage-program changes. But at least on United and Delta, the minimum-spending requirements for achieving status “pretty much pull the rug out from the mileage run, the premise of which has always been getting the most miles for the least dollars,” Winship said.

Michael Sommer, a technology consultant, actually sounded a bit nostalgic about this. “This is the last year you can really do a mileage run based upon just miles and cheap fares,” said Sommer, who this year has Delta’s highest status, Diamond, though he says he invariably flies on “the cheapest ticket possible.”

When I called him the other day at his home in Park City, Utah, he had come back the previous night from a year-end mileage run. His report: “I went Salt Lake-Atlanta-Tampa, and coming home it was supposed to be Tampa-Minneapolis-Salt Lake. But in Tampa, I called them up and said, look, I’m doing a mileage run, what do you have space-available that can give me a couple hundred extra miles? So they gave me Tampa-Atlanta-Los Angeles-Salt Lake. But then in Atlanta, the flight to Los Angeles was overbooked and they said, we need a volunteer to take a later flight and you’ll get a travel voucher. I said: ‘I’m it! I’m on a mileage run and you can route me anywhere you want.’ And they said, how about we send you to New York and from New York we send you to Salt Lake and give you a $500 voucher? I said, ‘Great, I’ll take it!’”

And believe it or not, despite the convolutions, in years to come, frequent fliers such as Sommer might well look back on year-end marathons like that and say: Ah, those were the days.