Michael Lehman thought he would be enjoying his retirement by now. The veteran CFO already retired once, in 2002 at age 51, from a 15-year career at Sun Microsystems. Called back to the same role in 2006 when his successor stepped down, Lehman saw the company through peaks and ultimately valleys, helping to sell the technology giant to Oracle in January. Once that deal closed, he says, he expected to finally be able to devote more time to biking, golf, and travel.

But the life of leisure has been postponed. Instead, Lehman has gone back to the future in a sense, taking on the CFO role at Palo Alto Networks, a pre-IPO company in the enterprise network-security industry. The company announced his appointment last week.

Lehman says he met with the recruiter for the position, David Nosal of Nosal Partners, intending only to pass along the résumés of a few candidates for the position. But Nosal persuaded him to meet the company’s management team, and when he did, “I got really excited about the company and its prospects, and my ability to make a difference here,” says Lehman.

Making a difference “can be quantified in different ways,” the 59-year-old notes, and he expects his opportunities to do so as Palo Alto’s first CFO will be different from those he had at Sun, whose annual revenues peaked at more than $18 billion in 2001 and remained over $13 billion before the recession hit. For one thing, “they’re looking to me to build infrastructure for the company to be able to grow and scale, and it’s interesting and appealing to be at the front end of these decisions,” he says.

Also, none of Palo Alto’s other managers, including CEO Lane Bess, have been executives at a U.S.-traded public company before, so Lehman’s years in front of Wall Street analysts should prove very useful. Bess says he “needed a CFO who knew how to navigate the ups and downs of the public market, and build the company for a trajectory of high growth,” and adds that Lehman couldn’t be a better fit.

Indeed, Bess admits that when Nosal first proposed Lehman as a candidate, he was skeptical that the veteran CFO would even consider the job. “I said to myself, ‘I can’t imagine he’s going to want to dive back into a flashback of Sun Microsystems,'” recalls Bess.

It’s not as if Lehman was hurting for money. In 2001 he made $37 million at Sun, most of it from options gains, making him the highest-paid public-company CFO at the time. According to Sun’s October proxy, Lehman earned $800,000 in annual base salary from 2007 to 2009 and stood to gain more than $4 million upon a change in control, such as Oracle’s takeover. (Lehman exchanged his Sun shares for $870,998 in cash at the time of the acquisition, according to InsiderScore.com’s read of Securities and Exchange Commission filings, and exchanged his Sun options for Oracle ones that currently carry a value of nearly $1 million.)

So what snared Lehman? “I think he got a glimpse into the excitement he saw at the early days of Sun,” says Bess. Lehman says that in fact, Palo Alto reminds him of Sun Asia, Sun’s Hong Kong–based subsidiary, where he was director of finance and administration from 1989 to 1990. “That was about the same size, with fast growth and relatively immature processes, but a great team,” recalls Lehman.

Palo Alto, which makes network-security tools targeted at helping companies monitor and control employees’ use of Web 2.0 applications such as Facebook and YouTube, closed $36 million of Series C funding in late 2008. It currently has about 1,200 customers, and the board and management are hoping to bring it public next year, says Bess. The CEO declines to disclose Palo Alto’s annual revenues, but he says his goal is for the company to hit $1 billion in revenues within five years.

Lehman starts at Palo Alto with between 25 and 30 people reporting to him through the finance, IT, and manufacturing functions he oversees, a far cry from the 3,000-strong staff he commanded at Sun at its peak. “I will have to build the team and will in many cases have to participate much more directly in some of the decisions,” he notes. Faced with the prospect of creating an income-tax strategy for the emerging company, Lehman says he realized that “at Sun, I would have turned to our VP of tax and said, ‘Please go figure it out.’ Here, I am the VP of tax.”

For now, then, the golf and travel will have to be confined to vacation time. “I probably don’t have another 22 years in me, like I spent at Sun,” says Lehman. “But there’s a lot I can still accomplish.”