Fitness for a dummy like me

I am a 23yo male (Indian), currently living in London. Over the years, I feel I have become successful according to my age and achieved my dreams, purely from a career perspective. I work in IT, have an exceptional job in an MNC, I live at a very good location geographically, and have the money and freedom to do whatever I want. However, I can’t help but wonder how much better my life would be if I could commit to becoming a fitter version of myself.

With a diagnosis of ADHD, and fitness never being a part of my childhood, it has always been difficult for me to start and stay consistent with losing weight and becoming healthy.

Every time I try asking anyone about fitness, I always feel so behind. The way people explain fitness often overwhelms me with all the things I need to do just to get started. I’ve also had a weird relationship with food because I’ve always put work before my health (stress eating, picky eating, etc.). To be fair, health was not even in my top 10 priorities.

That changed after I had an accident last year where I hurt my back and couldn’t move for two weeks straight. That’s when I realised that maybe having all the money in the world is not where I want to be, and that I want to focus on becoming a healthier version of myself.

So my question is: assuming someone has the worst eating habits of all time, has never exercised consistently except for some basic football, walking/jogging and biking to work back in the day, what would you recommend they start with if the goal is to lose belly fat and eventually build enough stamina to run marathons? I also want to build muscle, but for now, I want to start slowly to avoid going too hard too soon. To anyone who has committed and seen results, what’s your story?

Here are some metrics about me:
- Age: 23
- Gender: Male
- Weight: 70 kg
- Height: 5'5"