Joshua Weinstein always wanted to be an airline pilot, but the industry was in crisis when he started college in 2002, so he became a middle school teacher instead.

He loved that job, but after a decade of flying in his free time at a cost of tens of thousands of dollars, Mr. Weinstein began hearing more about a looming pilot shortage and left the classroom in 2018 to pursue his dream. It worked: In January, he started training to fly for ExpressJet, which operates regional flights for United Airlines. But the coronavirus pandemic, which devastated the airline business, could thin the ranks of pilots by the thousands and has already put the nascent careers of people like Mr. Weinstein on hold.

“The worst part right now is that the only thing we know is that nobody knows anything,” he said. “There’s uncertainty. We just don’t know what happens next.”

For years, flight schools, airlines and experts encouraged people like Mr. Weinstein to become pilots. They promised young recruits a job that was lucrative and secure because thousands of pilots in their late 50s and early 60s would retire in the coming years and demand for travel would continue growing. The profession is still stacked with older aviators, but airlines are expected to make deep cuts in the coming months, and the pilots most at risk are those who are just starting out.

While air travel has recovered somewhat, it is still only about a fourth of what it was last year, according to airport security data. Most experts say the recovery will be slow and uneven because of a patchwork of travel bans and the unpredictable nature of the pandemic. The recent surge in coronavirus infections has already forced some governors to delay reopening their state economies and to shut down bars and other businesses. If cases continue to increase, as some public health experts fear, air travel could become a lot less appealing.

To prepare for that uncertain future, the nation’s largest airlines are stockpiling billions of dollars in cash. If ticket sales do not recover soon, American Airlines, Delta Air Lines, Southwest Airlines and United have said they could resort to job cuts as soon as Oct. 1, the first day when airlines are free to eliminate jobs and reduce hours under a stimulus law that Congress approved in March.

Airlines could lay off, furlough or reduce the hours of tens of thousands of pilots, cuts that would disproportionately fall on those who have less union seniority and training. Major airlines have already stopped hiring pilots after posting hundreds of openings in the first quarter of the year, according to Future & Active Pilot Advisors, a consulting firm.

Several companies are offering buyout packages to avoid deeper cuts later. Southwest, for example, has said it isn’t likely to need more than a thousand pilots on its payroll, according to its pilots union, and is offering several years of partial pay and benefits to those who agree to leave the company temporarily or permanently. Delta warned last week that it could furlough nearly 2,600 pilots and is offering early-retirement packages.

Some pilots said the turmoil was nerve-racking, but those who have been in the profession for a while have come to expect it.

“You kind of know going in that aviation has high highs and low lows,” said Lisa Archibald, 41, a Delta pilot and volunteer with the airline’s pilot union, the Delta Master Executive Council. “You do it because you love what you do.”

Like Mr. Weinstein, Ms. Archibald arrived at the job by way of a detour. After graduating from Purdue University’s School of Aviation and Transportation Technology, she was hired to fly at American Eagle, which American Airlines owns. But the job started days before the 2001 terrorist attacks, and she was furloughed after just a few weeks.

About a year later, Ms. Archibald found a job piloting corporate jets, which she did for 15 years. She joined Delta in May 2017.

Unsurprisingly, pilots are passionate about the profession. That’s why they spend years in grueling training programs, trying to rack up the minimum flight hours and credentials needed to become airline pilots, at a cost of up to $100,000, not including the price of a college degree.

Mr. Weinstein, 36, estimates that he easily spent between $50,000 and $70,000 on flight training, offset by what he earned working at the flight school and teaching middle school in New Jersey over a decade. At ExpressJet, first-year pilots earn a minimum $36,000 a year.

Many pilots borrow tens of thousands of dollars to pay for their training, loans that can take years to pay off. While veteran pilots at the big airlines can make as much as $300,000 a year, starting salaries at regional airlines can be as low as $30,000, according to Future & Active Pilot Advisors. Most pilots typically spend several years at a regional outfit before making it to a larger carrier.

The median salary for the country’s nearly 125,000 airline and commercial pilots is about $121,000, according to federal data.

In recent years, airlines were so worried about finding enough pilots that they took steps to secure a steady supply of workers. United, for example, said in February that it was buying a flight school in Phoenix in anticipation of a need to hire more than 10,000 pilots by 2029.

Boeing’s chief executive said last year that the pilot shortage was “one of the biggest challenges” facing the industry. Global demand was growing so quickly that airlines would need to hire 645,0000 pilots over the next two decades to keep up, about 131,000 of them in North America, Boeing predicted.

Historically, most airline pilots cut their teeth in the military, but the armed forces have increased the minimum years that pilots must serve, said Mike Wiggins, the chairman of the department of aeronautical science at Embry-Riddle Aeronautical University. As a result, for the past decade or so, most new airline pilots have been civilians.

The shortage was long in the making, but has been delayed time and time again, first by the 2001 terrorist attacks, then by the Great Recession and the Federal Aviation Administration’s decision to raise the mandatory pilot retirement age to 65 from 60. The pandemic will likely push it further down the road.

“A lot of those dynamics are still going to be there,” Mr. Wiggins said. “Obviously, the growth is not there right now. But the retirements are still going to be there. Time marches on.”

Mr. Weinstein knows those dynamics well. He had wanted to become a pilot since he toured a cockpit when he was 6, but his timing always seemed off. He started college just after the Sept. 11 attacks and graduated just as the Great Recession took hold.

He finally landed a piloting job this year, but was told in May that ExpressJet was pausing training for him and dozens of other new pilots. So Mr. Weinstein has returned to working at the flight school where he learned to become a pilot, offering virtual instruction and flying when he can.

He and Ms. Archibald have been in touch with people in their former careers, waiting to see how the next few months shape up and how their employers sort out what and how many jobs will remain after the federal stimulus ends.

Whatever the outcome, Mr. Weinstein said, all that effort has been worth it.

“I have something to show for it because I did make it to the airlines and I did get hired and I did achieve that dream,” he said. “And so part of me says not to regret a single moment of it, because I put my mind to something and I did it.”