The shortest endurance race is the one on Super Speedway. It covers just 150 miles (100 laps) and requires just two pit stops. However, I wouldn't want to do it in A-spec. All the cars are really fast, and of course the track is really boring. If I were to ever A-spec an endurance race, it would have to be one of the ones in which the AI vehicles use road tires on road cars, not race cars. Race cars may handle like they're glued to the road, but they're extremely difficult to control with the precision necessary to win races. I B-specced nearly every race in the game that required racing tires.
So yeah, just do one of the shorter endurance races where you can use a road car. They're much more forgiving. You could even try doing 1000 Miles! It's long, very long, and it's made longer because you're only allowed to use older (thus slower) cars. But it's fairly easy because you can usually easily outmatch all your opponents. It's not a very good value for the money, though. You put in lots of time and the reward is disappointing (you get 900,000cr for winning all four races, but the prize car is just some old Alfa).
If you want the fastest way to make money endurance racing, use the Fuji 1000km race. B-spec that and it'll take around 5 hours to complete, and that's if you leave and never come back to speed up the time again. Your reward is 750,000cr plus a car that you can sell for over 700,000cr. It's the best way to make money with minimal input and time investment. Unlike the DTM method, doing Fuji over and over again can be done when you leave in the morning for work or school, and before you go to bed. That's how I made enough money to buy every car in the game.