For Metro to maintain today's level of bus and rail service over the next 20 years, the regional transit agency will require more federal and local money, higher fares from additional passengers, cuts in costs and new sources of revenue beyond fares and subsidies, according to a plan approved yesterday.

This view of Metro's future was part of the agency's first long-range plan, a draft of which was adopted yesterday by a committee of the Metro board.

The full board is expected to approve the plan Dec. 13.

When Metro was planned in the 1960s and 1970s, the agency's emphasis was solely on building the 103-mile system and creating and operating the regional bus system. Now, however, with the system's remaining 13.5 miles due for completion in the next 12 years, the agency is facing a new set of challenges and wants to redefine its role.

"We now are at a mature stage," said Peter Benjamin, Metro's planning director. "The focus now is on, 'Where do we go from here?' "

As sketched in the plan, the Metro of the future may be extended beyond the 103 miles, or linked with other rail and bus lines operated by public and private interests.

It will be larger and more cumbersome to operate, with 83 stations instead of today's 64, and 875,000 daily rail trips, up from the current 529,000 trips.

Metro will be older, too; rail cars, some of which are 14 years old, will have to be replaced and repaired, and the average age of buses will be higher than the current 10.6 years. Buses are designed to be used for six years.

The agency also will be hit with two potentially expensive federal mandates to reduce pollution from diesel buses and to expand access to the buses by disabled persons. The committee created a regionwide committee yesterday to figure out how to comply with the latter requirement by January 1992.

As a result of those changes, the plan said, Metro will need hundreds of millions of dollars. Replacing 300 rail cars will cost $1.25 billion, and repairing the system will cost more than $100 million a year over 20 years.

Even with a 5 percent fare increase every two years, local government subsidies to Metro will more than triple, to about $1 billion, each year by 2010, the plan said.

With financially strapped local governments unlikely to shell out that kind of money to Metro, board members are looking at ways to cut costs and raise revenue, including annual fare increases pegged to inflation.

The plan also stresses the need to attract more riders by making the system easier to use, and it says that Metro should seek new revenue sources, such as selling air rights around stations to developers.

On top of the money shortfall, the plan said, Metro will have to deal with a shrinking pool of potential employees, making it harder to attract qualified workers.

Of particular concern to board members is the proliferation of proposals for links to the rail system. Connections have been discussed in Northern Virginia and suburban Maryland.

Many of the groups proposing such links have not discussed the implications of adding their system to Metro's, and the plan approved yesterday suggested coordinating these efforts.