By the time he was in his late thirties, Chris Williams was enjoying the life of a rising young executive. A vice president at a tech-consulting firm, he attended high-powered meetings, flew business class, and brought home a six-figure salary that supported his family’s comfortable, suburban life. But despite the trappings of success, Williams was stalked by a niggling doubt: that rather than being on a sure road to shimmering golden years he was on a slow, steady slide into professional mediocrity.

This scenario might be familiar to you. And if experts, who estimate that the average person’s career peaks when he’s 45 (this is when you’re likely to reach the height of your seniority, and the way you handle your career at this age will determine whether your earnings stagnate or continue to increase), are right, then you should be worried. What you’re doing—and earning—at that age will have a powerful impact on the rest of your life. To go on autopilot in your thirties and early forties is to squander what’s known as the career sweet spot. It’s as damaging as dropping out of high school.

“You make your career in the decade between 35 and 45,” says Brian Drum, a New York executive recruiter. Or as Stephen Viscusi, the author of Bulletproof Your Job, puts it, “That’s when you make your ‘fuck you’ money, the cash you’ll need to secure your future.”

To have any chance of your professional path leading upward, you’re going to have to start treating your career with the seriousness of a fighter preparing for a title bout. Most guys get on the career ladder in their twenties. By their mid-thirties they’re pretty much set on a particular course. Then they take their foot off the gas. This might be the biggest mistake you can make. Doing fine in your thirties isn’t good enough if you want to be breathing the rarefied air of the executive office in your fifties.

The consequences of complacency—leaving the office early to coach your kid’s Little League team, sticking with a company that’s in a declining industry, excusing substandard work because you’re tired—can be drastic. “I had a client once who just blew it,” says Sarah Cardozo Duncan, a career strategist. “He made the decision to move away from where he was living for his wife and family. The only problem was the job prospects in his new town weren’t half as good as the job he left behind. Now he’s struggling, while the people he used to work with are in the $500,000 category.”

But the 10-year push that starts in your mid-thirties isn’t just about securing the country house and the ski trips to Gstaad; it’s about establishing yourself so that eventually you’ll not only be earning the big bucks and making the big decisions, you’ll be in a place where your professional status is assured. That way the rest of your working life will be a smooth run into the home straight rather than an anxious grind.