Imagine running a marathon in blistering heat each day for 3 1/2 weeks.

That's the idea behind the Tour de France, the Super Bowl of cycling, which is heading toward the finish Sunday with a dash down the Champs Elysees. Since July 1, the world's best cyclists have been riding up to nine hours a day, often reaching speeds of more than 55 mph.

"I've treated athletes in a lot of different sports," says Gerard Zaragoza, trainer for the Renault-Gitane team. "But cycling in the tour has to be the world's most physically demanding sport. You can burn 10,000 calories in a day, and then you have to do it again the next day."

The riders put up with this agony because the tour is one of Europe's most glamorous spectacles. In France, where bicycling is the equivalent of baseball in America, rural roads are clogged each weekend with amateurs testing their limits; the tour cyclists are heroes.

As many as 18 million roadside spectators cheer the cyclists on. Up to 100 million fans follow the race on television.In terms of crowds and coverage, tour organizers boast that this makes the race the world's largest annual sporting event.

"The tour is part of the national patrimony," says Robert Taurand, the tour's secretary general. "Just like the Chateau de Versailles."

The tour is cycling's ultimate test. The route covers varied terrain, from the lush, flat farmlands of Normandy, Brittany and the Gironde, up to the hills of Midi and the peaks of the Pyrenees and the Alps, and forces the rider to be both a good sprinter and climber.

The racers also must have tremendous endurance to cover a total of 2,312 miles with only one day of rest.

They live a stark life. After finishing the day's stage, they return to a hotel, eat, shower, and usually watch a half-hour of television before falling asleep. There are no nights on the town, and wives and girlfriends stay at home.

"After racing, I'm so tired," said veteran Lucien Didier of the Renault-Gitane team. "I'd like to party, but I know what is coming the next day so I don't."

About a third of the 140 cyclists who began the tour will not finish. Some will have aggravated minor injuries or been hurt in spills. Others simply will have become too tired.

"You must really suffer to race the tour," Taurand said. "The race is a 4,000 kilometer battle."

This year the battle has been particularly fierce because an injured right knee forced cycling's superstar, Bernard Hinault, a 28-year-old Frenchman, to withdraw before the race. For the last five years, he so thoroughly dominated the event that the major excitement was who would finish second.

"Hinault discouraged competition," says Pierre Chany, a columnist with the French sporting newspaper, L'Equipe. "Without him, this year's race has been much more interesting."

The top riders have stayed bunched together and, from day to day, the leader's yellow jersey has been passed from one racer to another.

The favored veterans, two 36-year-olds, Joop Zoetemelk, a Dutchman, and Lucien van Impe, a Belgian, have been displaced by new younger faces. Van Impe, a former champion, did not show his usual form in the mountains, although he won the 19th leg Thursday, and is now third in the overall standings. Zoetemelk was found to have taken hormones and was penalized 10 minutes.

The refreshing rookies include three Frenchmen, Pascal Simon, Laurent Fignon and Jean-Rene Bernaudeau, and a Spaniard, Pedro Delgado, all in their twenties. Simon won the hearts of the entire nation with a rare display of courage.

After gaining the lead in the Pyrenees, Simon, 26, fractured his left shoulder blade in a fall after his bicycle was bumped. Despite the injury, he continued to lead the tour for most of the second week and into the beginning of the third.

Doctors treated him with heat, ice and laser beams and taped his shoulder each day, but Simon's ability to maneuver with his left arm was limited and his pain was obvious.

He reported he could not sleep through the night, and that the pain worsened daily. But he said he did not want to quit while wearing the yellow jersey.

Monday, Simon coasted onto the shoulder of the road and put his feet on the ground. He later expressed remorse that he did not continue.

"I'm very sad and very sorry," he said, "but the pain is too strong."

Simon's departure gave the lead to Fignon, 22. Today, Philippe Leleu of France made a lone break from the pack and held the lead for 113 miles to win the 20th leg. He finished more than nine minutes ahead of his nearest challenger. Fignon finished ninth and held on to the yellow jersey as overall leader. This is Fignon's first tour, though, and his lack of experience has observers saying that the race is still wide open.