Building Base

I define base training as training at 50-70 percent of VO2 max, which is a comfortable, conversational pace.  If that’s what you’re asking.  We all need this once, or possibly twice a year, every year.  In other words, you don't do it once at the beginning of your career.  You will always incorporate base training at some point in the year.

Generally, coming from a running background (as I think you are, too), building a base means low intensity training that is meant to prepare the body for more intense stuff.  It also will develop the parts of fitness that respond well to low intensity training (e.g., Heart, lungs, muscle capillarization, etc).    Generally, base work is done before you start a formal training program.  Speed work is pretty important to include during the right times of a training program each year.