Everyone knows Aesop's Fables, where a Tortoise and a Hare have a race (long distance, apparently) and in the middle of the race, the Hare, who was ahead, decided to take a nap. The Tortoise, on the other hand, was going at a slower pace... but did not stop. The perseverance paid off, and the slow Tortoise beat the fast Hare. The moral, 'slow but steady wins the race' works well for those in training because it is the individuals with the perseverance and steadiness that works their way up. Should the training be done quickly, there's a chance that the individual would hurt themselves, but by gradually building up the training, it gets the job done.
Sometimes, people are obsessed with the here and now and want that instant gratification. Ads on the internet tells me the quickest way to lose fat, or how to gain muscle... things that sounds great... but the thing is that it doesn't give you the perseverance needed for a lifestyle of growth. After finishing an endurance race, I'll take a break for a little bit because I need the recovery time... but because of my lifestyle, I would get back into running, and little by little improve the amount I run, getting to a point where I run, improving my time, but mainly to continue my lifestyle, working towards maintaining my health.
Progress does not happen overnight. Progress happens through every little thing you do, taking steps towards your goal. Being like the Hare and taking a long break fails you to reach the goal, while being like the Tortoise and plodding on little by little does in fact get you to the goal. Today, I'm taking a day off, making more time for me to just chill and hang out with family, as well as resting up my muscles from the hard work from the last few days. There are times to run, and there are times to rest, and as long as resting doesn't become an everyday event, it's good for the body rest, so that it can build itself up and become stronger.
Remember the constant movement of the water in a stream. As it moves over the jagged edges of a rock, its constant movement slowly smooths down the rock until the jaggedness disappears and what remains is a smooth river rock.