Now, he has a point. Thirty minutes in to this odd little, startlingly predictable, unambitious comedy and I was thinking how is Pegg ever going to get me to feel like he deserves this girl? Predictable used to be a critical barb. Now, all movies – excuse me – mainstream movies, unlike 90% featured on this site, are predictable by definition. The audience cannot be challenged or short changed or made to feel anything other than "They got what they came for." This says a lot about audiences or rather the perception of audiences. I adore the Billy Wilder quote: "An audience is never wrong. An individual member of it may be an imbecile, but a thousand imbeciles together in the dark - that is critical genius." The audience this evening laughed in all the right places (I may have smiled once or twice). As I have mentioned ad nauseum, we know today's mainstream movies are predictable but if you can get to London from Cardiff without using the M4 then the movie may just manage to entertain. There are enough off the beaten track deviances to make Fat Boy mildly diverting. It's just that after Shaun of the Dead and Hot Fuzz, you see Pegg's name and think, hmmm, something offbeat and worth tuning into (despite his forays into fan-boy mainstream with turns in Doctor Who and - of all things - the bloated cruise-controlled Mission Impossible series). I even enjoyed watching him in the sitcom Hippies. But this one is so by the book, it feels like a pair of spectacles at bedtime.

No hoper, Pegg, jilts Thandie Newton, at the altar, pregnant with his son. Note, Dylan Moran is Pegg's best man (Moran does irresponsible, unreliable, skanky slimeball in his sleep) so either there weren't a lot of best mates to choose from or he and Pegg have something very special. I had expected at least a Nick Frost cameo but you can't have everything. Maybe he was too busy on the crushingly ordinary Hyperdrive sitcom. There are enough cameos, thank you very much. Pegg's career prospects are not bathed in heavenly Jacob's Ladders. He's a security guard at a woman's lingerie shop (let's get Izzard on to the unfairness of that) and cannot even run down shoplifters. His relationship with his 'son', played very well by Matthew Fenton (not at all precocious and over the top) is Pegg's only saving grace. Why those attributes together should be a baseline supposition for child performers is beyond me but to my shame I add it as a personal prejudice anyway. It must be all those US kids' movies I have been watching for a decade. Yes, the only ace Pegg can play is how good a father he turns out to be which rather begs the question, why would he bother to enact paternal duties if he wanted to run away so badly in the first place? Well, of course, he's changed his mind and now wants her back.

The mark of a clever romantic comedy is finding a likeable and sympathetic foil or adversary for our hero to overcome. If the hero's competitor for the heroine's affections is a prize one jerk then, it's straw dogs, easy to knock over so no great connection is made with the movie. One of the worst offenders of this crime was Crocodile Dundee. Or (and this is the bit that really grinds my teeth down to the Headless Horseman's proportions) have these marketing bastards really taken over the asylum? Is this such a normal staple that audiences would miss it if it were not present and correct? Well, this is where Fat Boy does a curious thing. In gradual stages, Hank Azaria shows us his true assholic credentials (won't let Matthew control a toy boat, gets impatient with the six year old and treats Pegg like the no hoper he is). Now normally, this would just be so predictably dull that it would not be worth watching. But, and here's Pegg's neat little reverse – during Azaria's smarmy confident arrogance, Pegg paints himself as a little, spiteful child, just as annoying and silly as his rival. You kind of hope Thandie ends up with neither of them and cosies up to her ovens in which she cooks her produce (she owns a bakery).

The script was in development for a number of years with director (and 'Friend') David Schwimmer attached. Financing eventually came about which necessitated a London shoot but still, like Four Weddings etc., the sop to the great American unwashed has to be offered. It's such an obvious gesture that you'd think a smart audience would reject the nonsense out of hand. But of course they don't. Hungry people will eat anything. So while the oddly talent-challenged and make up model Andie McDowell became the reason for Four Weddings's finance (who'd have thought?), the die was cast or rather the leading cast member was American no matter where your movie's set. Hank Azaria brings a few bits of interesting baggage with him. After all we are talking about Mo, Apu and Chief Wiggum here. If you have no clue who these fine upstanding folk are then I recommend a book "How To Cook For Forty Humans"...

Azaria, the voiceover talent aside, still manages obnoxious American, quite serviceably. There is (as you may have seen in the trailer) a literal dick swinging contest between the rivals and it's all rather low brow and of course, my audience loved it – "You see my point?" Indeed. But in the end (ooer, missus), do we care about the insufferably childish Pegg and his infantile (OK, adolescent) challenge to run the London Marathon – oops, it was to be the New York Marathon and oops, they couldn't get (or didn't want) permission to use the London of the species so guess what? Some bright industry spark said "Hey, let's advertise the shit out of 'global-sport-brand' and name the marathon "The 'global-sport-brand' River marathon"! I could weep. Movies have dived beneath hell and are looking for another Dark Lord to service because Satan doesn't cut it anymore.

Simon Pegg would die in a marathon. Hank Azaria convinces that he would comfortably finish. So how could this absurd premise work? Well, it does to a degree. You have your obligatory half comedy based, half Rocky-based training montage. Dylan Moran has bet his physical well-being on his friend completing the race. Pegg's landlord, a delightful, full bodied turn by Harish Patel, uses a spatula to great effect. Given the cast, I was delighted our Indian character had no reason to say "Thank you, come again." His gorgeous daughter delivers the ultimatum – finish or you're out, no-rent paying boy. So begins the race of his life.

I won't spoil it if you enjoy this light, airy fare because there are a few nice surprises along the way. Yes, Run, Fat Boy, Run does (on its predictable course) stop off at some all to obvious points but there is enough charm to carry it over the finish line.