Tom McCarthy is the feel-good story of the week, the 75-year-old high school principal who paid pennies for a horse that just happened to blossom into an entry in today’s 135th Kentucky Derby. Yeah, that’s what Wayne Lukas and Bob Baffert thought. At least until Tuesday’s trainers’ dinner, when they sat down with McCarthy and were reminded why you can’t believe everything you hear around a racetrack.

“A schoolteacher woke up one morning and had a Derby horse,” Lukas said, reciting the alleged story line. “Nothing could be further from the truth. The guy was training quarter horses in the ’50s.”

A few men leading horses to the paddock this afternoon know what they’re doing. This is McCarthy’s Derby debut, but together, Lukas, 73, Baffert, 56, and Nick Zito, 61, account for 80 entries and nine champions in the Run for the Roses.

“Stay with the gray-haired guys,” is Lukas’ advice today.

They’ve left Churchill Downs in successful years thinking this stuff is so easy and departed in difficult times wondering how they could have ever tricked themselves.

“This race will humble a lot of people,” Lukas said. “These young trainers will walk out of here Sunday morning shaking their heads. You watch.”

This year, Baffert was elected into racing’s Hall of Fame, but it wasn’t long ago that the media branded him as brash and disliked. That’s not the Bob Baffert who will saddle Pioneerof The Nile.

“I’ve learned to be competitive in a very quieter manner,” he said. “You keep thinking, ‘Oh, I’m just going to keep doing this,’ and then all of a sudden, I started bringing some really good horses here and I got beat a few times.”

Baffert won three times in six years starting in 1997, but he hasn’t finished in the money since and was out of the race the past two years.

Lukas’ 42 Derby starters are a record, and only six-time winner Ben Jones has trained more Derby champions, yet he failed to reach the starting gate since 2004.

Lukas’ horse, Flying Private, joins Zito’s last-minute entry, Nowhere to Hide, at 50-1.

“He’s as good as some of them I’ve run here, including a couple of them that won,” Lukas said of Flying Private.

Zito had five Derby horses in 2005, yet since then, he has saddled only two entries. Late withdrawals opened the door for Nowhere to Hide.

Zito knows the Derby drill and says that will count today.

“You know how to handle everything,” Zito said. “See, the whole thing is to keep your routine.”

McCarthy, the rookie, knows that because he shared a barn with Zito two years ago during the Derby and has studied Lukas’ Derby training records in detail.

McCarthy trains General Quarters, who has generated more attention than your typical 20-1 shot.

Many wouldn’t be surprised to see the horse challenge I Want Revenge (3-1), Dunkirk and Pioneerof the Nile (both 4-1) and Friesan Fire (5-1).

He first saw the horse at the 2007 Keeneland yearling sale, passed on him, then spent the drive home regretting it. Last May, he got a second chance when General Quarters popped up at a $20,000 claiming race at Churchill Downs. He bought the horse, who won the Sam F. Davis Stakes at Tampa Bay Downs, earned $604,000, and here they are.

“Yeah, I thought everything had gone by me,” McCarthy said. “I don’t think it’ll ever come my way again, so I’m going to take advantage of every minute of it.”