Remember Giuliani back in the run up to the 2008 elections? Widely touted as one of the big favourites early on, what with being a blue state Republican with a proven record (though admittedly also a tendency of shooting his mouth off). And then when push came to shove, his campaign didn't amount to anything once the primaries began.
Leading in the polls this early (and seriously, we aren't even in the year of the midterms yet!) doesn't mean anything except having wide name recognition. Which is an asset, to be sure, especially when most of what is said about you is positive, as is the case with Christie. But what matters is where he will stand two years from now, and who his rivals will be then. Being an early frontrunner can be a curse as well as a blessing.
Anyway, I'm sure he'd make a decent president, and if he can get the nomination without having to scare off independents and moderate Democrats too much, he should have a good chance of winning, though that's a big if. A lot will depend on the political climate on both sides by then.