This week marks the halfway point in the National Football League's regular season, and one of the biggest surprises is that all 28 head coaches who took their teams to training camp still have their jobs.

Coaches have been known to fail to last through the exhibition schedule. Just ask George Allen. Invariably, during a normal year, one or two of these gentlemen would have been fired by now.

Not that there are not several coaches whose jobs are in jeopardy.

Baltimore (1-6) cannot be too happy with Ted Marchibroda. Homer Jones is not doing any better in Cincinnati. Both clubs were expected to win more than they have.

San Francisco, on the other hand, is right where many observers figured they would be at 0-7, and Detroit's 1-6 record is understandable in view of the injury to quarterback Gary Danielson.

Injuries to two or three key players can be the difference between 5-2 and 2-5. And some of the coaches mentioned above probably have as many problems with the front office as they have on the field. A good general manager can make an average coach look brillant.

Certainly, two of the current candidates for coach of the year will be looking at each other Sunday when Philadelphia plays at Washington. Dick Vermeil steadily has been upgrading the Eagles until they are now 6-1, while Jack Pardee is making the Redskins (5-2) forget Allen.

Washington is among the teams I will dabble with this week, along with Oakland, New England, and New Orleans for a mythical $250 each, while Pittsburgh is a more serious $1,000 selection.

On paper, Philadelphia normally would be the pick as a 2-point underdog in RFK. The Eagles have been able to run on the Skins in their recent matchups. I look for Washington to cheat defensively against Wilbert Montgomery and gamble against Ron Jaworski having too hot a hand. This figures to be the Redskins' best effort of the season as they're coming off a four-game road trip emotionally high.

Ken Stabler should have a delightful fime picking apart the Jets' secondary. Oakland, a 2-point favorite, has become a strong contender in the AFC West.

New England's Steve Grogan always scares me. But Bob Griese is not throwing sharply for Miami, either. I'ii give 7 1/2 with the Saints' excellent offense at home against Detroit.

Finally, there is Pittsburgh. Don't ask what happened to the Steeler defense the last two weeks. I don't know. Whatever, it was, Craig Morton should be able to solve the problem. Pittsburgh will apply strong pressure to the Denver quarterback Monday night, and Morton can't stand pressure. Give the 7 1/2.

In other games this week Las Vegas lists Buffalo 6 1/2 over Baltimore, Tampa Bay 5 over Green Bay, Cleveland 3 1/2 over Cincinnati, Dallas 10 1/2 over St Louis, Minnesota 1 1/2 over Chicago, Houston 2 at Seattle, Kansas City 6 over the New York Giants, San Diego 1 over Los Angeles and Atlanta 4 over San Francisco.