NEAR THE end of the 10th and final hour of the NBC mini-series "Marco Polo," the hero, having undergone interrogation in a Genoa prison, tells the priests and fellow prisoners standing around him, "As God is my witness, I've told less than half of all I have seen."

It sounds like a threat; is he going to tell even more? No one could be expected to sit through another syllable. Handsome, ambitious and occasionally spectacular as it is, what NBC has on its hands with "Marco Polo" is a majestically pictorial stifferoo. It is as visually beautiful as anything ever produced for television and as dramatically inert as a Cinerama travelogue.

The 10-hour, four-part, $30 million production about the exploits of history's most famous tourist premieres tonight with a three-hour episode at 8 on Channel 4, then continues nightly at 8 through the three-hour conclusion on Wednesday. Those who stick it out for the duration will get what has become fairly par for the course, mini-series-wise: two hours of sumptuous and enthralling material spread out over an agonizingly protracted 10-hour marathon.

And yet. And yet. As photographed by Pasqualino De Santis, who shot the current theatrical import "Three Brothers" and other works by Francesco Rosi, and as scored by the most innovative and eclectic movie-music composer in the world, Ennio Morricone, "Marco Polo" dwarfs traditional television in that it truly is something to see--and something to hear, and occasionally, something to get blearily, blissfully lost in. In the last half, the script is polka-dotted with the same kind of star-struck Eastern mysticism bandied about in the NBC mini-series "Shogun," but again, there's something attractive about it. Despite the arduous pacing, many viewers may find "Marco Polo" a peaceful center in the otherwise shrill and cacophonous video landscape, and it sure is pretty.

It is bewildering, though, that director Giuliano Montaldo, who wrote the script with Vincenzo Labella and David Butler, was allowed to take things this slowly, to drag them out so painfully. "Marco Polo" is the first such film to be shot in, and made with the complete cooperation of, the People's Republic of China, but viewers don't get so much as a peek at China until the beginning of the fifth hour, which is the middle of part II (tomorrow night). Much of the first episode is given over to protracted wrangling among clerics about the propriety of the trip. They talk till they all but drop.

Montaldo is no whiz at getting the show on the road and, once on it, he has difficulty keeping it there. "Marco Polo" begins with a prologue that would test the patience of a catatonic. It is 1298, we are in the Genoa prison, and a prisoner and historian named Rustichello (David Warner) is being beseeched to tell the tales that his fellow inmate Polo, captured during a sea battle, has told him.

The church does not look kindly on such stories because they challenge traditional beliefs, it is stated--and restated, throughout the series (only part III is mercifully free of these oppressive framing sequences). So there is hemming, and hawing, and a little more hemming, and finally a priest barks out at Rustichello, "WILL you begin?", the way Gleason used to explode at Carney on "The Honeymooners." At last the flashback starts and we are into the story: "It all began in the year of our Lord 1254 when Marco Polo was born in Venice. . . ." Time and again the production bogs down in enervated rhetorical colloquy, and one wishes that testy priest would reappear to tell everyone to get on with it.

Most of the early scenes about young Marco's Wonder years are of negligible interest and impact. Anne Bancroft begins the parade of star cameos with a ludicrous quickie as Marco's mother, abandoned years earlier by his father, Niccolo (Denholm Elliott), a merchant who went off to seek a fortune in China. Bancroft has an especially cumbersome and rib-tickling death scene in which she weeps and weeps prodigiously and then, when a tear appears in her son's eye, says, "Don't cry, Marco."

Perhaps even more out of place is Sada Thompson as Marco's auntie, who adopts him when Bancroft makes her watery exit, and who sputters things like, "Marco, Marco, why are you always running away?" Later, Burt Lancaster has a high time as Pope Gregory X, John Gielgud does well by the doge of Venice, and the woefully overexposed John Houseman gets another chance to growl and a chance to wear a particularly silly haircut, as a surly bishop. In the third and fourth episodes, Leonard Nimoy subtly and cagily plays Achmet, a wily and manipulative Turk.

And in the same episodes, Asianactors cast in key roles make much more robust, emphatic impressions than most of the frivolously deployed guest stars, especially Ying Ruocheng as a Kublai Khan truly to be reckoned with; Junichi Ishida as his epileptic son, Chinkin; Soon-Teck Oh as the militant Wang Zhu; and the beautiful Agnes Mei Li Chan as Mai Li. An Occidental love interest is conveniently, and suspiciously, dreamed up for Polo, who seems immune to the Oriental lovelies all about him; as played by Kathryn Dowling, who has a voice very similar to the sweetly foggy sound of Eva Marie Saint's, the character gains an emotional credibility lacking in the script.

Two crucial weaknesses enfeeble the production, in addition to its length. One is its overall innocuousness. This can probably be traced to the fact that "Marco Polo" is sponsored in its entirety and was coproduced by the Procter & Gamble Co. Also, there was doubtless a conscious, and understandable, effort not to offend the Chinese, who had the powers of censor over the script, which thus ended up with the blandess of diplomatic bureaucratic prose. In part II, the Polo party encounters a supposedly rowdy tribe of Mongols in North China; they're so squeaky-clean they could be extras at Disneyland. You'd think there would be some talk of pleasures of the flesh among the tribesmen, one womanizer in the bunch, or maybe a sheepizer. Something. But no. "Marco Polo" is generally as lustless as it is listless.

The other key problem is the lead actor, the faint and ingenuous Ken Marshall, who suggests a long-lost Bottoms brother on the one hand (Timothy, maybe, at his dewiest) and the hazy gaze of Keir Dullea on the other--not exactly the old one-two. Marshall, reportedly cast on orders from P&G, in no way physically embodies what one would imagine Marco Polo to be like; he's white bread without even a bit of crust, an old-hat flower child in drippy full bloom.

All the worst tendencies of the actor, the screenplay and the direction are embodied in a scene from part II in which Marco, having been injured in an avalanche, wakes up in a Tibetan monastery where he sees a levitating lama, talks karma and nirvana with a tour-guide monk, and hears the immortal phrase "Oo lah lo ho" from a friendly Buddhist. "All is merely illusion," Marco is told, much as the hero of "Shogun" was told that nothing quite exists. It's scenes like this that don't quite exist.

The real star of the film is its scenery, and "Marco Polo" offers a visual banquet the likes of which have never been seen in a dramatic production for television. Even the skies that stretch over the Chinese plains seem exotic and otherworldly. In part III, Chinkin takes Marco to see the Great Wall, and it really is the Great Wall; it was already 1,000 years old. There's nothing particularly remarkable about a scene in which Marco and his girlfriend kiss and take a trip down river to a cave, but the setting makes it breathtakingly exquisite.

Television shows us far too little of the world, especially of those parts of the world where no wars or demonstrations are being waged, and so the sights to behold in "Marco Polo" are terribly welcome and may be reward enough for viewers tenacious enough to last the whole trip. An incredible array of costumes was designed and executed for the series by Enrico Sabbatini, enhancing the visual richness of the experience.

If only the story and screenplay were the equal of the trappings. In this 10-hour trek there is hardly a single powerful dramatic scene, save maybe one near the end, when the old Khan attempts to mount his horse, finds he has grown too feeble, and Marco lends him a couple of hands up. It took a perverse kind of talent to drain the story of so much of its intrinsic pizazz.

The length, of course, is the killer. In February, NBC Entertainment president Brandon Tartikoff announced that early footage of "Marco Polo" so thrilled the network that it was expanding its air time from a planned eight hours to the present 10. No one could be naive enough to think such decisions actually have anything to do with esthetics, or the demands of storytelling. Cost-effectiveness is the great dictator here.

It isn't as if the story demanded so much time for a proper telling. "Shogun" was 12 hours of TV, but a two-hour version--presumably coherent--was released to theaters abroad, and is distributed domestically on the home video circuit. The CBS thriller "Salem's Lot" was allotted four hours of air time originally, later played as three, and is now showing on HBO at a length of one hour and 50 minutes. At half the length, "Marco Polo" would still be too long, but it's the nature of network television to inflate these things beyond reasonable levels in the interest of balancing books and maximizing the potential audience. The first part of "Marco Polo" is so slow-going that the network may find it has outsmarted itself; the sound of a million dials clicking may be heard in the land.

Those who resist the temptation to wander will find much to satisfy wanderlust, however, and it is impossible to come away from "Marco Polo" without at least a slightly enhanced sense of the period depicted and the cultural diversity being celebrated. Perhaps the closing shot of "Marco Polo" should have been of men walking on the moon, since there isn't all that much distance between his small step and theirs. "Certainty is a dream," Marco is told in China. "We are all journeying toward the great unknown." Every now and then, "Marco Polo" really makes you say "Gosh."