I'm currently around 20% bodyfat. I eat healthy, drink when thirsty, sleep 7.5-9 hours, walk/bike as much as transport and do "convict conditioning" workouts 3 days a week. Outside of that, I don't "workout" but I am active in chores, hiking, etc.
All my friends are extremely invested in fitness and all sub-15% bodyfat. I am very disciplined but I try to focus more on nutrition than fitness (I cheat too much!).
I'd love more insight and if you had studies on this I'd love to learn more about efficient/minimal exercise.
If you're consistent and do intense aka 75-85 max heart rate at least once for 30 minutes per week that makes a real difference over time. Lower intensity requires a lot more because the body needs to adapt to stress.
Traditionally you see stuff like : http://www.heart.org/HEARTORG/HealthyLiving/PhysicalActivity... and people say I don't have time for that. But studies find meaningful change happens at under 1/2 their recommended levels.
Being economically secure including heath insurance and living below your means cuts out some major issues, but beyond that has minimal benifit.
Partners / children can be a boost or stress point depending on the person, but loneliness is an almost universal drag. Thus seek and maintain positive friendships.
Heath is somewhat up to the whims of fate, but a heathly diet, good sleeping habits, and minimal exercise can maximize what you have to work with.