C. McClain (Mac) Haddow, chief of staff to Health and Human Services Secretary Margaret M. Heckler, usually comes to work at 7 each morning and often stays at least 12 hours.

But now, without giving up that post, he is taking on a second job -- with some of the worst headaches in the department. Haddow will be acting head of the Health Care Financing Administration, which runs the $100-billion-a-year Medicare and Medicaid programs. Haddow plans to fill the job until a full-time administrator is nominated and confirmed by the Senate to replace Carolyne K. Davis, who resigned last week after holding the job since 1981.

How is he going to be able to do it? And why would he want to?

"I'll go home when the work's done," said Haddow, a former aide to Sen. Orrin G. Hatch (R-Utah) who joined HHS after a two-year term in the Utah legislature. "I'd like to keep it down to 12 hours a day. But my wife has accepted that I am a hard worker and stay long hours."

He noted that as chief of staff he already is involved in many major decisions on Medicare and Medicaid, so the extra burden won't be as a great as it might seem.

Some of his duties as chief of staff "will be farmed out" to Carol Bauer, one of Heckler's former congressional aides (and wife of Undersecretary of Education Gary L. Bauer) who has joined the department, and David Rust and Hannah Sistare, two other top Heckler aides.

Haddow, who turns 35 on Friday, said he's taking on the HCFA job because "the secretary didn't want any wobble" in the two huge health-care programs. "Often it happens that when a permanent chief of an agency leaves you get wobble" until a successor is chosen.

In this case, it may take up to seven months before a successor to Davis is chosen, investigated and confirmed by the Senate, Haddow said. The leading candidates for the job are Sheila Burke, longtime health aide to Senate Majority Leader Robert J. Dole (R-Kan.); William Roper, special assistant to President Reagan on health policy; William Ryan, an executive of Johnson & Johnson; Sheila Smythe, an executive with New York Blue Cross and Blue Shield, and Helen Cummings of Hospital Corp. of America. Haddow is not a candidate for the job.

In the next few months, HCFA is facing the department's toughest immediate problems. "In the time between now and the swearing in of the new HCFA chief, we need control and leadership" to handle those problems, Haddow said. He said his post as chief of staff should give him considerably more authority to deal with HCFA policy issues than someone who is simply acting administrator.

One of the big problems facing HCFA, Haddow said, is to set the final rates the government will pay hospitals and other medical providers in fiscal 1986. The rates must be set by September, and hospitals are fighting the administration's plan to freeze rates at 1985 levels.

The department is also trying to determine the best way to include payments for hospital construction and other capital costs under the new Medicare prospective-payment system.

At present, rates for each hospital stay are set in advance for Medicare patients, based on their diagnosis. Capital costs are not included in the calculation and the government simply reimburses the hospital for Medicare's share of whatever the hospital spends on capital outlays. HHS is studying how to include capital costs in the basic payment -- perhaps by simply adding an extra amount to the prospective payment rate.

"The issues," Haddow said, "are how to phase in the new capital payment and a decision on the amount of the add-on: 7 percent?"

Still another big issue is how to pay doctors in the Medicare program. The department is looking at several possibilities, including some form of prospective payment or some form of general fee schedule, to hold costs down.

In addition, Haddow said, several important regulations are awaiting action. These include a controversial proposal to change the tight requirements for staffing nursing homes, whether Medicare should pay for heart transplants, and what set of wage scales to use in computing Medicare hospital payments.

Haddow's office sports a pair of sneakers on the floor and a big banner embroidered "Brigham Young University, No. 1, 1984 National Champions," commemorating his alma mater's great football year. But he laughs when asked if he played football there. "At Brigham Young," he said, "I was on the debate team."

For exercise -- he's managed to lose 30 pounds dieting over the past few months -- he golfs or plays basketball at his church or in a pickup game on the tiny half-court in the department's fitness room.

One of the problems he's had to confront since he assumed the HCFA job is his mother. "We're both conservative Reagan Republicans, but on some issues she's more liberal than I am. She's been telling everyone I'm going to expand Medicare."

He had to call her up and ask her to stop.