Los Angeles — If only Adam Sandler and Seth Rogen hadn’t gotten in the car.

If only they hadn’t left Los Angeles, where everything in “Funny People” was going so well, and driven north to Marin County, where everything falls apart. Judd Apatow would have had his most mature, accomplished film to date.

Instead, the last hour or so meanders interminably, its tone wavering all over the place, leading to a quickie conclusion that feels pat. And that is such a letdown when you consider the strength and ambition of the material that preceded it.

(Shot by Oscar-winning cinematographer Janusz Kaminski, Steven Spielberg’s longtime collaborator, “Funny People” also looks a lot more polished than the first two films Apatow directed, “The 40-Year-Old Virgin” and “Knocked Up.” You could think of it as “Sandler’s List.”)

But Apatow should have maintained his focus on the friendship that forms between Sandler (as superstar George Simmons) and Rogen (as aspiring stand-up Ira Wright) as well as the established comics and wannabes that surround them. Instead, he has his characters make an unnecessary road trip in search of George’s long-lost love — with both George and the film losing their way.

We first see him as a rising comedian, courtesy of home movies Apatow injects of a young Sandler, his real-life roommate long before either of them made it big. Today, George has a thriving career based on his popular stage act and silly crowd-pleasers. The movies, which have titles like “Sayonara, Davey!” and require him to appear as a merman or grown-up in a baby’s body, are a dead-on parody of the kinds of dreck on which Sandler has built his empire. You have to give him credit for so gleefully poking fun at his worst work.

But then George learns he has a terminal disease. Suddenly, his perspective on everything duly changes, from the stacks of scripts waiting to be read to the random women willing to jump in his bed. He still wants to work but lacks his former enthusiasm — hence his interest in Ira, who’s young and hungry the way he used to be and who reminds him of a purer time. After seeing Ira do a set at an L.A. comedy club, George hires him to be his assistant, joke writer and friend — and the only person he initially tells about his illness.

Apatow handles their scenes together with surprising delicacy and zero sentimentality; Rogen, usually a bellowing bear of a screen presence, has slimmed down on the outside and nicely underplays it on the inside. The moments when he and George are harshly tooling on each other or confiding in one other are some of the film’s most appealing, despite the heavy subject matter; so are the ones in which George, Ira and other comics bat around ideas and hone their craft. (Jonah Hill and Jason Schwartzman play Ira’s roommates.)

Cameos from the likes of Ray Romano, Norm MacDonald, Dave Attell and Sarah Silverman as themselves add to the authenticity, but it’s a scene with Eminem — someone who’s experienced his own share of highs and lows — that crystallizes everything “Funny People” is about.

From there, though — not to give away too much — George drags Ira with him to chase after the one that got away: Laura, who’s now married and living north of San Francisco with her rich, Australian husband (Eric Bana). Laura is played by Apatow’s wife, Leslie Mann; the couple’s daughters, Maude and Iris, play their girls, Mable and Ingrid.

Ostensibly, this overlong segment is meant to demonstrate the kind of traditional, satisfying life George might have had if he’d made different choices; instead, it plays like a self-indulgent showcase of Apatow’s family at the expense of cohesion and momentum. And there’s nothing funny about that.