This is something I have strong feelings about! I have always been good about exercising as I walk a lot and run through the local hills which I actively like.
For me it was about diet I always ate what I thought was a healthy low fat, high carb, diet. A bit over a year ago at 47 I got all kinds of bad blood test results back, borderline type 2 diabetic and high cholesterol and my BP was too high. This was despite running 4 times a week and not really being overweight (my BMI was just below it but only just). They wanted me to start taking all kinds of drugs.
Turns out I don't process carbs well. I cut back big time on carbs (I rarely eat grains, sugar or starches now), all my blood tests and BP are totally normal without any drugs and I weigh what I did when I was 21 years old and doing a lot of cycling. I feel great. I still run regularly but can run faster and longer then a year or two ago. What has really blown me away is that I eat a lot more fat than I used to (nuts, avocado and dairy mostly) but my cholesterol has fallen way down.
My personal view is that there is not a single best diet for people out there and that we are all different. If you get to the stage where they want to start you on a bunch of drugs think carefully about what you are eating and think about changing things up it sure made a difference for me!