+1

Base riding lasts entirely different lengths for different people. Some need 1,000 miles, others 4-5,000.

It also repeats. Every year, I (and others) go through a base-building phase. Since I've been riding for a few years now, this base for me is only a 2-3 months. As a new rider, you have no base whatsoever, so you need to just ride the first year or more.

Base miles is a period in your riding where you're "initializing" your body. The muscles, tendons & ligaments all need to be taught about riding. They need to stretch & lengthen, and get stronger so they can absorb the stress riding puts them under. Then, they need to "remember" it. You're basically changing your body's physiology.

The riding in base building is usually just steady. Not specifically slower for slower's sake, but a steady pace: no sudden accelerations. The rides are usually also longer duration. It's almost all in zone 1 or 2 (zone 5 being your maximum effort), usually in the small chainring (but not always). It's also generally used to shed weight gained in the off-season.

If you begin to ride hard without a suitable base, where you make some serious efforts, at best you won't be able to go as fast/long as you desire, or may hurt yourself at worst. "Riding hard" could mean going as fast as you can, violent accelerations, or riding up a long or steep mountain passes/hills.

Last, rigid adherence to any of these principals isn't for everyone. Usually only racers stick to something as specific as this. So, at the end of your 1st year, re-evaluate: do you want to race? Do you want to adopt a regimen like this? Riding may lose its luster for you if you do. Or, you may enjoy the structure and racing. Personality wins out.