First, there were singers. Then there were song stylists. Then there were rock stars and all hell. Public television looks back longingly to the middle ages of popular song, the stylistic era, with more than three hours of "American Pop: The Great Singers," tonight at 8 and again tomorrow at 4:30 on Channel 26.

The production, by New York's WNET, kicks off a 30-hour fund-raising marathon that seeks alternately to delight and infuriate viewers into a donating mood. Among the highlights of this hysterical effort are a couple of classic Laurel and Hardy pictures set for the wee hours of the morning and, tomorrow afternoon, the unbeatable MGM musical double-bill of "The Band Wagon" plus "Singin' in the Rain." These are the two best Hollywood musicals made in the 1950s, and lots of area Betamax machines will surely be getting some exercise.

"American Pop" is a program of slovenly, cloddish, deplorable style and occasionally intoxicating content. It is such a formless hodgepodge of material and attitude that it bounces from the ditzy scat singing of Sarah Vaughan to the oddball religioso pop of "A Simple Song" from Leonard Bernstein's "Mass" to another minesweeper medley by Ethel Merman to the K-Mart camp of Johnny Ray reprising "The Little White Cloud That Cried."

THIS is the greatest of American popular music?

Also interspersed with the more or less live performers (taped in January at The Plaza Hotel in New York) are insultingly edited film clips ostensibly paying homage to singers who either could not or would not appear on the program. Some of these clips are framed in an electronic logo that teduces the film image to the size of a baseball card. This is the kind of program that makes you want to find the producer and mug him.

But there are plenty of unassailable interludes as well, and the first song of the evening is the last one written by the Gershwins, "Our Love Is Here to Stay." sung by Tony Bennett. Bennett dominates the proceedings to a finally self-defeating degree; his mannerisms -- the pauses and the bantamweight jabs at the moon -- are beginning to smother the songs he sings. Yet there is a tradition being kept alive here, and one is hard put not to respect it. 'Jericho Mile'

"The Jericho Mile," ABC's Sunday Night Movie, at 9 on Channel 7, is a film long on running as opposed to a long-running film. Peter Strauss, as the convict hero who shocks prison officials by running a mile in under four minutes, runs and runs and runs, and the film makes a mad dash to nowhere.

Strauss plays a character called Rain Murphy, a name obviously chosen to sound like Randall McMurphy, the hero of "One Flew Over the Cuckoo's Nest," and he gets to be first a martyr and then a hero to his fellow convicts at Folsom State Penitentiary, where the movie was filmed. Some of the extras really are Folsom convicts.

Unfortunately, the Transparency of this gambit is inescapably ritualized; here is yet another story about "a winner," someone who forces himself "Rocky"-style to attain a physical goal and achieve self-love. The fact that the characted killed his father in a rage is made into a virtue, since we are told the father was a drunk who liked to beat up on the kid's sis.

This is taking convict chic to a zany new level, and Strauss, not so vividly remembered anymore for "Rich Man, Poor Man," does a hokey imitation of a street-wise dude whom the script assigns such dialogue as "What went down is what went down." That's by way of explaining why he offed pops.

Director Michael Mann's best moment is the brief prison yard montage that opens the film. Otherwise the picute of prison his film gives suffers considerably wheb compared with the foul-mouthed authenticity of the documentary "Scared Straight," which, by coincidence, will be rebroadcast by Channel 5 tomorrow at 10 p.m., right opposite the last hour and last few thousand miles of "jericho Mile." 'Weekend'

The last segment on this week's "Weekend," the NBC News magazine, is by far the best." "How They Find Fame and Fortune," produced by Beth Polson, casts an ho-so-aptly jaundiced eye at the teen-idol biz in Hollywood, where a magazine publisher named Charles Laufer orders the eyebrowplucking, tooth-capping and nose-faced tykes into Shaun Cassidys. They hope.

"Weekend" is always at its best when impressionistic and askance. The first two segments on the program -- one on a youth worker who used to be a heroin addict, the other on the plight of the aged in cramped Japan -- are both models of clarity and professional reportage, but they get a little luiling as they trod painstakingly on.

Cohosts and reporters Linda Ellerbee and Lloyd Dobyns remain a fun couple however, even through all the program's evolutionary cosmetic changes, pressure from network executives, and grumbling from cranky affiliates. Cranky affiliates? Who cares. There are few people who know or care less about television than the people who run television stations.

"Weekend" closes with an animated sketch about the declaration of World War III. It seems that the editors at a mythical metropolitan newspaper are so busy remaking sections with names like "Style" that they can't be bothered with the War. Sesides, there aren't any illustrations available. One needn't find this sort of gratuitous cheap shot valid to congratulate producer Reuven Frank on having the misanthropic gall to take it.