100%

When running at a 100%, you are sprinting flat out and can barely breathe. 

When working at a 100%, you are fully engrossed in what you're doing and are putting in the highest quality of effort, you are curious to learn anything and everything that will improve your delivery, and you don't have a single thought to spare to anything else. You might even forget you haven't slept or eaten.

80%

When running at 80%, you are still running very fast, your heart rate is quite high, and you can make grunting noises if you want to respond to someone, but not manage to say more than a word every few seconds.

When working at 80%, you are still fully engrossed, and you are putting in a very high quality of effort, but maybe you aren't going that extra mile to exceed all expectations. You'll still wind up when it's time to eat or sleep.

50%

Now, you are just about jogging. Your heart rate is just above resting rate, and you can comfortably have a conversation as you jog, though the other person might be able to tell that you are jogging (if you are on the phone).

You are attentive enough to not let anything or anyone catch you out, but you are nowhere near putting in your best quality of effort, you are not interested in finding out ways to improve your quality, you are just cruising.

20%

You are walking at a slow pace. Sure, it's better than sitting on the couch, but only just. If anyone calls you a couch potato, you can tell them you exercise but you know very well it isn't doing you a great deal of good.

You are doing just about enough to not get fired, you are more interested in the ant roaming around on your desk than in the work you are doing, you are on the lookout for the first plausible reason to let you take a break from the work.

0%

You're on the couch, maybe having a dream about running, if that.

You have tuned out and are waiting to be fired. You're almost egging your boss on to fire you by not completing tasks even when asked to specifically.

Of course, objectively, how you run or how you work at 80% might be how someone else runs or works at 50%.

That doesn't matter.

In the long run, if you are consistently finding yourself at 80% or higher, you'll eventually learn to cruise at that and that starts to become your 50% or even your 20%. 

Which is how you get better.

You can define what each of these levels looks like for anything that you're doing - your habits, your relationships, your health, your attempts at learning something or achieving something.

The only way to get better is to consistently turn up at an 80% or above.