The other New York marathon got underway Monday and though there may be fewer partici-pants and a smaller cheering section, the front-runners were easy to spot before the day was over.

It was the start of the showings of clothes for spring, 1980, by New York designers, the first clothes to arrive in stores for the new decade. And while it follows closely on the heels of the fashion race in Milan, Paris and London, the New Yorkers are not overshadowed by the Europeans, at least so far.

What designers on both continents seem to want to say is that tight, rigid clothes that were around last time they showed their clothes (and many are in the stores now) just won't do for warm weather. Softer, looser, more feminine and often shorter is the rule on both sides of the ocean.

The big difference is the Europeans are more experimental and more exaggerated, if for no other reason than that they play to an audience of thousands, grandstanding their designs on football-field size runways in three huge tents. If you want to make the point that you like short skirts, for example, you have to make them really short for the message to hit the back row.

The American designers are playing to packed houses in their showrooms . . . Bill Blass was the only exception so far this week, with a hotel showing. And while the Europeans are concerned about making headlines and selling to their boutiques, the American designers are in the manufacturing business, with their clothes interpreted into hundreds even thousands, and sold in a broad range of sizes all over the country.

What the New York designers had in common was that they are among the few real winners in a fashion season that some ar describing as "not great," with few exceptions, others calling it "difficult at best."

What has been selling are the very expensive things when they are interesting and new, such as Perry Ellis sweaters, other handknits (or those that look like handknits and thus look individual and expensive, which they are), Bill Blass' (and others) quilted coats. "If they love it enough they will buy it at any price," insists June Fitzgerald of Bullock's, a California specialty store chain.

What's not selling are clothes that look so familiar they might be hanging in the back of the closet. Particularly if they are very expensive. And several buyers put Calvin Klein, in the past always a winner no matter what he showed, as a disappointment this year.

Perry Ellis, who showed Monday, proved he is a master sweater designer, the best being snug-fitting short sweaters worn with flippy short skirts, or better still, soft trousers that cut off above the ankle. (Remember flood pants . . . ? These are that length or shorter and very fresh looking.)

He understands that women do not want to be bound in by tight, tight bottoms when the weather gets hot, but he provides one curious alternative, skirts that are padded below the waist at the hip. Even for those who don't have their own cellulite padding, it's a curious look.

Ellis calls them "farthingales," a hark back to the 16th century when paddings pushed out skirts like panniers. But that was before subways and supermarkets. "I can't help it, I just like the look of it," says Ellis, who has shaped his linen farthingales with a wad of organdy. Although some stores have already bought them that way,

Ellis admits that he'll probably sell more without the padding. Besides he says, you can wear them with padding and when you want to change, just take out the padding. And shorten the skirt.

Donna Karan and Louis Dell'Olio, the designers for Anne Klein, have just been bouyed by a season in which their things have sold well, particularly the angora sweaters, the two-button jackets, the loose-cut trousers, silk blouses. "Women are buying items," says Karan, "but it always has to be special so that the woman says, 'I have to have it.'"

So that is what Karan and Dell'Olio have done for spring . . . cotton sweaters, soft blouses, easy skirts (always at the knee), easy long trousers that will brighten and update what a woman already owns. The buyers loved it. t

Bill Blass, who in one short visit to Washington recently sold $45,000 worth of clothes at Neiman-Marcus, knows his customer well. He knows she has lots of dough and he knows how to make her part with it. His strong suit is grand entrance clothes and he can make a ruffle stand out so dramatically on a dress that even the ruffle haters of the world would be impressed.

He has spun off a few ruffles from his most successful styles of the current season, and has found new ways to douse clothes in ruffles for spring. And then he has done a group of short-jacket suits in navy and white that will keep all those ladies who want to make grand entrances at night out of the shadows by day.

For those not made for ruffles and tiers, there are strapless short dance poufs of ostrich feathers in bright colors or bicolored snakeskin jackets to wear over anything you own.

And all they cost is money . . . an awful lot of it. Prices for designer clothes generally are up about 15 percent again, hitched to the lower value of the dollar paid for imported fabrics and the highed cost of doing business such as labor mimimums and real estate taxes.