Advices for Someone on Early 20

It is hard.

First you need a purpose, a motivation. Not the kind you get on a sudden morning, but a visualization of how the end result is going to be. Because, the sudden motivations will go at the same speed it came to you. But many of these things needs a lifestyle change and lifestyle change needs consistency and for consistency you need to have something to hold on it.

Unless you have those "vision" to see yourself of, it is very hard to motivate yourself when you don't have results or if you're not motivated. Then again, many of these things I have heard in my early years but took the later half to figure out and settle on. So give it time, it will come.

There are plenty of books on the subject and I'd encourage you to just search and read.

Discipline as you note is essential in anything you do, work related or otherwise, there isn't much that I can add here that you can't already find via a few searches, for me it's always helped taking things one step at a time.

You need to train yourself to enjoy doing these things. If it is painful you will not do it.

When you exercise learn to appreciate how it makes you feel. Think to yourself "I am getting stronger, better looking, this is great!"

find peers who are into these things and push each other. another advice for your 20s, focus on making lasting friends and networking with decent people, way more important than anything else you do