You want run forever, go Toyota or Honda.
I'm really surprised niether have a 40mpg car (can't/won't do a "hybrid"). Not that it excludes either (generally they are still a bit proud compared to their competition) just an observation.
Longevity and durability imply a heavier frame and components than you'll find in a current 40mpg car.
I don't know anything about Honda Fits, but I believe the Civic is a bit larger and heavier. And Civics are among those that can go 300K with regular maintenance.
No doubt. The problem is when you compare comparably equipped vehicles, Toyota and Honda are more than a few dollars higher. For what I'd want, more like a few thousand plus. All I really need is A/C, auto and a very few other items but the standard features vary pretty wildly amongst manufacturers. Certainly still a consideration for the reasons you mention and others.
Since you mention all you really needs is A/C,Auto,and other small things I can tell you the Ford Fiesta S model with auto will be under 14K and will get really great gas mileage.
And unlike the Elantra,Soul,Accent, Cruze,it comes equipped with a spare tire,lug wrench,and jack.
I posted a while back about going to lunch with a co-worker that had just bought a new Elantra and we ran over a piece of metal which caused a flat,in the pouring rain no less,and that's when we discovered no spare came with the car.
In February I started pricing new economy cars and came extremely close to buying the Ford Fiesta SE but was finally put off because it's made in Mexico and has a timing belt that Ford claims is good for 100k,but I HATE timing belts severely.
What I finally bought was a left over 2011 Toyota Corolla LE that dealerships had a small sea of and for once Toyota had to deal.
Car retailed at $19,800 and I paid before TT&L 16,300.
Took a 3,000 mile trip in it in April and the lowest mileage it got was in a west Texas head wind of 25 mph and still got 32 mpg's.
Best mileage was from Springerville Arizona to Tucumcari New Mexico utilizing the cruise control and got a stunning 43 mpg.
No wind noise,smooth ride,and very comfortable seats.
Good luck and look at most cars out there.