President Reagan climbed back into the saddle today for his first ride on horseback since his cancer surgery in July.

"Americans like cowboys and that is the way many people think of the president," said one Reagan adviser. "Riding a horse is a sign that he's really back, that he's well again."

Since he arrived at Rancho del Cielo, his mountaintop ranch northwest of here, on Aug. 11, Reagan's first post-surgical ride has been the focus of intense media attention, particularly from television networks, which have kept long-lens cameras continuously trained on the ranch from a nearby mountaintop in hopes of a glimpse of the president on horseback.

Reagan, smiling broadly, arrived this morning for his weekly radio speech dressed in boots, western shirt and khaki riding breeches.

"Well, I think you've noticed the change of attire," he said to aides, who conveyed his remarks afterward. "Today's the day."

Reagan delivered a relatively bland back-to-school speech in which he criticized "value-neutral" education. And then he went off to ride a dark bay horse named Elusive Hobby.

By waiting until a slow Saturday in late August, the president enabled the White House press office to dominate a slow news day with the story that Reagan was once more on horseback and that all seemed right in his pleasant, remote world in the Santa Ynez mountains.

Visual events always receive primary attention in the Reagan White House, and the press office advised reporters early in the day that an official photo of the ride would be available by mid-afternoon. Officials knew it would be seen frequently on television and in newspapers during the weekend.

But the utility of Reagan's cowboy image goes far beyond the transient one of winning ink and air time in the weekend news media. The image underscores the persistent portrayal of Reagan as an independent and optimistic westerner whose life embodies the individualism he preaches.

One of the cowboy values is endurance, and Reagan's ride today was a message from the White House that he is recuperating well from surgery and is ready to resume a leadership role.

The president, who often seems unaware of policy complexities, understands the importance of this message. Before the radio speech, chatting with aides and fingering a blue baseball cap, Reagan took note of published reports that he looks pale and said this is only because doctors have ordered him to shield his face from the sun after a bit of cancerous skin was removed from his nose last month.

Reagan has been well-tanned since his days as a teenage lifeguard. He has boasted that he rarely needed makeup in his acting days because of his tan.

Reagan returned to the ranch from Los Angeles Friday while many top aides, including chief of staff Donald T. Regan, remained on vacations of their own. The president is scheduled to appear briefly at a party for the White House press corps in midweek but to make no public appearances until a Labor Day speech in Independence, Mo., en route home to Washington.

In his radio speech, Reagan said that excellence in education is forged in the classroom, "not by bureaucrats in the far-off city of Washington." He denounced "value neutral" education without mentioning his customary advocacy of school prayer and called upon students to improve their own education by demanding more homework.