While I spent the spring congratulating myself for my 12-to-1 future-book bet on Plugged Nickle in the Kentucky Derby, many friends have tried to temper my enthusiasm with a sobering reminder.

Even my mother, who is willing to overlook the grossest character defects in her son, could not resist pointing out, "Andy, you know your record in the Derby has not been . . . impressive."

This was a delicate bit of understatement. I bet against Secretariat. I threw out Foolish Pleasure. I detested Seattle Slew. I scorned Affirmed.

The first horse I ever bet here, Holy Land, fell in the 1970 race. And over the years, my selections seemed to be afflicted by some mysterious and dreadful curse.

Logically, there must be a reason why the world's greatest handicapper has been so consistently wrong about America's greatest race. Finally, I think, I have discovered that reason. I have begun to understand the Kentucky Derby, and having done so, I think that Plugged Nickle's prospects on Saturday look golden.

When a handicapper studies a race, he ordinarily concentrates his attention on the most recent performance of each horse. If he is a speed handicapper (as I am), he will judge them according to how fast they ran. But in the Derby this is much too narrow a focus.

To win at 1 1/4 miles on the first Saturday in May, a horse ought to have months of Derby-oriented preparation. Every Derby winner since 1972 has been trained by an astute, major-league horseman. Everyone has gone through an orthodox series of major prep races.

It has been relevant whether these horses have run fast while winning. Spectacular Bid's last prep for the 1979 Derby was alarmingly slow. Affirmed ran slow races throughout his whole winter campaign (while arch rival Alydar was running fast). Seattle's Slew's times were consistently mediocre.

What mattered was that these horses had gone through the right kind of conditioning program and had done pretty much what their trainers asked them to do. They won the Derby because of their total preparation, not because they ran one good prep race just before the main event.

A horse could not get better, more thoughtful preparation for the Derby than Plugged Nickle has.

"I started planning on the first of January," said veteran trainer Tom J. Kelly. "It's something you do turning in bed from two to four at night. You've got to figure out how to have him at his tops rather than overdone."

The schedule Kelly drew up was classic: a six-furlong allowance race, a seven-furlong stake, two major 1 1/8 mile stakes, and finally a mile workout at the start of Derby week. It was not too much, not too little. Plugged Nickle responded by doing everything Kelly wanted, winning all three of his stakes races with authority.

Few of the entrants in Saturday's field have had anything resembling this kind of textbook preparation. The Derby won't be won by horses like Genuine Risk, who has had only one tough race all year; or Withholding, who has raced once in two months; or Rumbo, who came here from California because of an 11th-hour decision by his owner.

The two rivals who do seem prepared to challenge Plugged Nickle are Rockhill Native and Super Moment.

Rockhill Native has run approximately as fast as Plugged Nickle this year; although his record is tainted by two inexplicable and dismal performances in Florida this winter.

But when Rockhill Native has scored stakes victories this season, he has done so after everything has gone his way. He won the Everglades Stakes at Hialeah by sitting behind two leaders engaged in a suicidal speed duel. He won the Blue Grass Stakes when nobody challenged him for the early lead.

He has not yet displayed (to me, at least) any guts, any ability to overcome adversity. On that basis, Plugged Nickle has the edge.

But there is still one nagging question about both of the favorites: Will they like the Derby distance? Neither has accelerated at the end of 1 1/8-mile races in a manner that suggests he will relish the 1 1/4-mile distance. If both of them are faltering in the long stretch at Churchill Downs, then Super Moment may beat them.

Super Moment's overall record is hardly dazzling; he has won only one minor stake in his career. But he got thorough conditioning this winter at Santa Anita, then came east for the Blue Grass Stakes and closed strongly through the stretch to finish two lengths behind Rockhill Native. He will like the Derby distance.

I can see Plugged Nickle in a duel with Rockhill Native on the final turn, dispose of his challenge as they enter the stretch and open a clear lead with an eighth of a mile to go. Then I can see Super Moment begin to accelerate, gaining on the leader with every stride. In case that scenario develops, I may be perched on the roof of Churchill Downs with a high-powered rifle.