Have you ever had an assignment due for school or a big project with a looming deadline at work? And so you work yourself super hard in the days leading up to the due date, submitting the assignment or project just in time, but then in the days after the deadline, you find it increasingly hard to focus or start on another project because you are just so exhausted? Your brain needs a break! Heck, you could probably benefit from an entire vacation. And if you were to just continue plugging away, your work would probably get worse and worse in quality. Well, this same principle explains what it means to "run slow to run fast".
If you want to show up on race day and run your fastest and best time possible, you can't be running your fastest and hardest throughout the week in the days leading up to the race. I mean obviously, right? Because you will be exhausted on race day! Just look to some of the best and most elite runners to find out that they save their speed for speed drill training days and race days. But for other training runs, and especially long distance endurance runs, low and slow is the name of the game. This helps build endurance and stamina but saves your legs and body from excessive fatigue that can add up quickly and result in a decreased running performance when it is time to really "show up".
There is a time and place for everything, and there is especially a time and place to run fast and hard and a time and place to run slow and steady. You certainly will not lose your speed, endurance, or fitness level if you choose to run slow. In fact, more often than not, you SHOULD be running slower. Your 5k or 10k race pace should be ran at once a week, particularly during a tempo run. But the other days of training? You should strategically be running seconds to even minutes slower than your goal race day pace.