Answer: It's easy on your muscles
Explanation: I think easing into an exercise program or schedule isn't just about building up endurance because that would only build up endurance slightly, the best way to build endurance is actually in terms of breath and heart rate is to push yourself after you run out of stamina, you most of the time ease into it because of your muscles, because if you start out full speed on your muscles you'll hurt it by pulling it, tearing it, or straining it, so you let them strengthen themselves so that you can progressively go faster and faster