While striking United Airlines pilots walked picket lines at National Airport yesterday, George Cusick, a retired truck driver from British Columbia, said he felt his stomach tie up in knots as he became convinced his vacation was falling apart.

But moments after some quick rescheduling by a United agent, Cusick and his wife, Theresa, were on their way to the gate, shocked at the ease of their departure.

"I thought there'd be thousands of people we'd have to push through," Cusick said as he left the nearly empty United counter. "This was beautiful, no problem."

The Cusicks were not alone in their relief.

While the United pilots' strike caused some inconvenience and widespread changing of travel plans yesterday, most travelers with United tickets at National and Dulles International airports did not confront the massive lines and short tempers caused by other recent airline strikes.

United officials said they had prepared for the strike for months, assigning dozens of extra employes at each of the Washington area airports. For most of yesterday, the lines at the United counter at National Airport appeared only slightly longer than usual and at times the lines disappeared.

"We're probably overstaffed right now," United spokesman John Philp said at 2:30 p.m. as a small crowd of ticket agents descended on each new traveler to give advice.

"We're absolutely amazed at how smoothly it went," said Larry Dunne, United's BWI sales manager. "It's unlike anything we expected, even with all our contingency plans."

Many of the 350 United reservations personnel at the regional "rez" office in McLean had worked around the clock calling ticketed United travelers to make alternate plans.

"We're taking the initiative, and we're not just waiting for them to show up here" at the airport, Philp said. Most local passengers were taking a few remaining United flights, or flights on Ozark, Republic, Eastern or Piedmont, he said.

Travel agents said the worst problems involved travelers who bought inexpensive United tickets under a pre-purchase plan such as the "ultrasaver," then switched airlines and found the new one did not honor the reduced fare. Many of those passengers have to travel on a standby basis to avoid paying the higher fares, travel agents said.

United canceled 20 of its 30 regularly scheduled daily flights in and out of National yesterday, keeping five departures to its main hub airport, Chicago's O'Hare, and five arrivals. United also canceled all 26 daily arrivals and departures at Dulles and all 24 at Baltimore-Washington International Airport. United plans to resume one daily trip each way between BWI and O'Hare starting today, officials said.

Plans are to maintain this trimmed down schedule for the near future, Philp said.

"We want to be consistent, not having a lot of trips today and none tomorrow," he said. "We view this as an irregular, regular operation, like a really bad snowstorm."

Meanwhile, striking members of the Air Line Pilots Association, who started picketing at 6:30 a.m. yesterday after a midnight strike declaration, said they were prepared for a long strike.

Their yearly salaries average $80,000, and they're striking to protest United's plans to pay new pilots about half that.

Lloyd Gentry, a 16-year United pilot, said some people had taunted the strikers for being well-paid. "I sympathize with that," Gentry said. "We're not striking for our money, but to make sure United continues to attract quality pilots to our cockpits."