For my own purposes in justifying a trip by air, I think about the difference compared to driving. Time and fuel required to make the flight compared to time and fuel required to make the drive. Then divide fuel economy for the plane in half because the gas costs more per gallon.
Here to nearest shopping mall is 120 road miles or 84 air nm. 2 hours driving at 60mph average and 6 gallons of $2.50 gas ($15 total) with no ado on either end. Plane goes 130 knots at 9 gph, so it takes 40 minutes and 6 gallons of $5 gas ($30 total) plus preflight and ground transportation at the destination.
Here to nearest real metropolitan area is 650 road miles or 450 air nm. Figure 90 minutes of total stops in the car and average speed while moving of 70mph, and it's 11 hours on the road. At 20mpg it takes 32.5 gallons of $2.50 gas, about $81. The plane goes 130 knots at 9 gph so it takes 3-1/2 hours and 31.5 gallons of $5 gas, about $158.
Trying to convert apples to oranges, it's like driving 180 mph without stops, traffic, road construction or other conditions, or police and getting 10 mpg after adjusting for gas prices. But you have some overhead that doesn't depend on the distance that slow down your equivalent speed.
A half hour on each end of the short trip and you end up with 1:40 point to point compared with 2:00 in a car. Equivalent driving speed is 72 mph and you don't have any real difference in number of meals eaten on the road. This is a flight for its own sake. For the long trip, it gets you to 4:30 flying versus 11:00 driving. Equivalent driving speed is 145 mph and you can keep normal meal times without eating on the road. This is a flight that trades dollars for a real chunk of time, at an exchange rate that I feel really good about anytime I can accept the chance of having to drive or stay home if the weather precludes flying.
In other words, you can answer the original hypothetical person's question with: The farther you go, the faster it ends up being.
But I'm still going to tell people "150 miles per hour and I get to use every one of them all the time."