The thing to remember is that however hard it is, you always just have to push yourself just a bit. You don't have to walk up x hill or run x distance or go at x pace. Your body decides for you, in the moment, what you should do.
If you pick something that you can do now, that pushes you (walk up the stairs/walk uphill for 30 seconds), and do it a few times over a few days, you will find that quite quickly, it won't be so much of a challenge anymore. Always keep the level of challenge the same, so, as soon as it gets easier, up the dosage (walk up a bigger flight of stairs/walk uphill for 40 seconds) After a while, you'll find that 30 minutes walking is fine, so you can throw in a 5 second run a few times. Then when that's fine, a 10 second run.
There's a lot of rigidity in training programmes, but as a very beginner, the most important thing is to learn how to listen to your body, and pace yourself. It'll save you many an injury, and it'll stand you in good stead in a couple of years when you're running a marathon
The first time you go out with the sole intention of 'training' will be the hardest. Get it done this weekend. It gets easier from there, and you get to decide the level of difficulty.