Despite representing a swingy suburban county, Astorino is very conservative (anti-choice, anti-marriage equality) while Cuomo remains broadly popular despite infuriating progressives, and his war chest is beyond enormous. Indeed, early polling shows Cuomo defeating Astorino by an average margin of 62-23. New York is simply a very blue state and is only getting bluer.
So what's Astorino's parlay play here? Even if he can somehow hold Cuomo to a "respectable" margin, he wouldn't have a much better shot at another statewide office somewhere else down the line, barring a wave year or a massive Democratic screw-up. As for the House, Astorino's home of Mount Pleasant is in the blue-leaning 17th District; if Democratic Rep. Nita Lowey were to retire (she's 76), he'd make a compelling candidate there, but it's still not a good seat for Republicans. Astorino could also conceivably carpetbag elsewhere in the Hudson Valley, but there's no looming open seat that's really begging for him.
Another, very different alternative would be for Astorino to try to land a media gig—before entering politics, he was a radio host and producer and still keeps a foot in that world. Losing a bruising statewide campaign for governor certainly isn't the easiest way to interview for a new job. But even after he loses, Astorino will still have three more years to serve as county executive, giving him plenty of time to plan his next steps, whatever they may be.