It is not that hard. It isn't difficult to stop eating the processed foods that lead to deteriorating health, you just have to decide that is how you will eat. This problem with "diets" is that it implies they are temporary and that once you have finished it you can go back to eating all the shit you used to.
For me I decided I would design my diet to exclude processed food and include the nutrient dense food I like, including the skin off the chicken(yum!). I'm a really busy person so when I cook, I cook a lot and freeze it. Then home cooked good food is about 10 minutes away.
They myth about fat being bad excludes the knowledge that you actually need it to carry water insoluble nutrients through the cell walls so that nutrient dense foods sustain your wellbeing.
The longevity of a geek/nerd career is closely tied to this. Awareness, cognition and being able to think deeply are all affected by what you eat so I structured my permanent diet to incorporate nutrient dense foods early in life. If you can avoid processed food your own instinct will guide you when you shop for food.
As for fitness, I am 90kg in my 40s and surf the same waves with my body that most people use a board for. I train several martial arts and still whoop guys in their 20s. The physical activity makes me a better programmer because I can think clearly.
The biggest secret about diets is you don't need them if you don't eat processed foods (including crappy sugar drinks) and focus on unprocessed food. Plus it doesn't need to be "organic", just fresh.