They really ought to let you filter people by driving time rather than distance as a crow flies on dating sites.
I imagine that'd either require a ton of Google Map's api calls (do those get expensive?) or doing some kind of in-house map routing that's probably not resource cheap.
But you don't have to know about weighted unidirectional graphs to know Long Island is harder to get to than Philadelphia from any point in New Jersey.
I guess you could just cache the driving times between major cities and use that to make a reasonable guess for places between those towns. Not good for trip planning, but good enough for "Do I need to buy a tank of gas and plan bathroom breaks to see this hunny?"