At the blast of a 105mm howitzer, the ninth running of the 26.2-mile Marine Corps Marathon will begin today at 9 a.m. near the Iwo Jima Memorial.

More than 12,000 runners will flood the streets of Arlington and the District of Columbia before the leaders return to the memorial at approximately 11:15 a.m.

Metro transit service will begin its usual Sunday schedule at 10 a.m., not 6:30 a.m. as reported yesterday. In past years, Metro had extended its morning hours to accommodate the runners and spectators.

Chicago had its marathon two weeks ago, and New York had its race last week, but all the hoopla and attention surrounding those two commercial events are not part of the marines' race. Rather, they have kept their marathon as traditional as others were before race directors began paying runners appearance and prize money.

The major reward for the more than 60 percent of this field who will be running a marathon for the first time will be finishing, and receiving a T-shirt and a completion certificate. For the others, a well-organized effort by the marines, a cool, overcast day and a personal best time would be reward enough.

Dean Matthews of Atlanta is favored to win the Middendorf Trophy, named after the former secretary of the Navy, Ambassador William J. Middendorf. Awarded to the overall winner, this prize is one of more than 300 given to division winners. Matthews, who won the 1981 race in a course-record 2 hours 16 minutes 30 seconds, will be running his second Marine Corps Marathon. He and Henry O'Connell have the best time, 2:16, of today's competitors.

O'Connell, from Kensington, is a veteran of 10 marathons. Michael Hurd of Great Britain, the 1980 champion, also is running today. He posted a 2:18:31 in beating 7,794 other competitors in the fifth running of the race.

The woman's course record of 2:41:48, set by Jan Yerkes in 1980, might survive another year. Beth Dillinger of Blacksburg, Va., is favored among the 1,710 woman entrants, having a marathon best of 2:46. Patricia Deuster of Silver Spring has a best time of 2:48.

Ken Archer of Bowie will try for his fifth triumph in six years in the wheelchair division. Archer, 35, won in 1979, 1981, 1982 and 1983, and holds the course record of 2:21:11, 17 minutes off his best time.