Juggernaut II, aged but ageless, won the 60th running of the Middleburg Hunt Cup today, beating favored Pidou II by 10 lengths with outsider Mr. Jam third another length behind at the finish of the 3 1/2 -mile, 16-jump timber steeplechase.

St. Denis, who had won the Middleburg Bowl here two weeks ago, was fourth with Squarely Rich fifth. Chatex ran out and was pulled up early while Amberson Place fell at the 15th jump.

The Jug, top timber horse in the nation in 1979 and "still the class horse of the circuit" at age 12, according to Pidou's rider, Randy Waterman, made it look easy as he covered Glenwood Park's course in the excellent time of 6;34 2/5, 3.1 seconds behind his winning time in this race two years ago at the Middleburg Spring Race Meeting. Only Annual Meetin in 1973 has run the race faster than the rangy chestnut.

"He ran just the way we wanted," said Jack Fisher, 17, son of owner John Fisher and a student at Brooks School in Andover, Mass. He flew here just for the race.

The Jug was content to stay off the early pace set by Pidou II, a fellow French-bred jumper who had returned to the circuit just two weeks ago with a fine victory at Upperville. Pidou II had been a big winner last spring before an injury retired him for the year.

Pidou was challenged first by St. Denis, then by Mr. Jam and by Amberson Place during the middle stages of the race, which drew a crowd of approximately 12,000.

Going into the long swing into the 15th fence, Fisher made his move and was comfortably on top as they cleared that jump.