Start with something easy and establish a rule that won't ever be broken. If you break a rule once, you'll lose the fight.
My rule, for example, when I started to train more:
- start with 10 crunches every morning and evening
- increase by 2 crunches every day
- no exceptions
When you are at ~2 months in, you can add weight training to it to get stronger.
Additionally, find a sport that you can do once or twice a week that is FUN to do. By FUN I really mean it. There's no point in doing sports if you don't enjoy it.
If you enjoy playing batminton, go for it! If you enjoy table tennis, go for it! If you enjoy Kung Fu, Krav Maga, or whatever ... go for it!
Sports isn't about reaching goals, it's about having fun while doing it. Otherwise, you will not overcome the struggles. Your brain needs a reward, and enjoying sports helps you keep wanting more of it.
You cannot outrun a fork.
Reality is more complex than memetic one-liners.
The obese people and sedentary office workers don't and would need to train for months to be able to out-run a single piece of cake on a regular basis without injury.
The phrase is good advice for that group of people.
To quantify this, on a weekly long ride they may consume 300g of refined sugar, far far beyond the often recommended limit of 20g of sugar per day.
So perhaps the sedentary person would benefit from walking or leisurely cycling to work a whole more than by trying to restrict their diet