I guess not smoking, no drugs and regular exercise. I really enjoy what I do - the challenge of achieving an analytical objective with limited resources has always given me great satisfaction.
How much exercise, and have you always been physically active?
[I'm more than halfway to your experience but definitely missing this aspect.]
About 1 hour in the gym 4 times a week with a mixture of anaerobic and aerobic always setting goals and constraints on my heart rate (using a Polar monitor). At least that is what I was doing until about 2 years ago, when my personal life changed for a number of reasons. Now, I mainly concentrate on walking 3 or 4 miles a day.
Thanks, that seems like a sustainably attainable goal (and to adapt and not overdo it).