Personally I’m not much of an endurance guy these days. I “ran” my last marathon in 2001 and have never looked back, but I realize there’s plenty of benefit to pushing hard for more than 20 seconds at a time.
If you want to get a good indication of your aerobic capacity consider giving the 6-minute test a shot. Odds are your average pace in the test will be very close to your velocity at V02max, which is know to be one of the best predictors of endurance performance and is a useful measure whether you’re a marathoner, basketball player, rower, or cyclist.
The test is simple: run as far as you can in 6 minutes. Make sure you measure the distance accurately. Not a runner? No worries. Then jump rope (count your revolutions), cycle, row, or swim. With training your score should improve. If it doesn’t, scale back to 3 minute intervals, but run (cycle, swim, etc.) at the pace you established for 6 minutes.