^^^ This. It takes me up to ten miles to warm up. Until the legs are warmed up, the feeling is just like you describe, my legs feel heavy and I'm breathing hard to maintain a slow pace. Once warmed up, my HR is able to rise without a perceptible increase in effort, and I'm going faster while my perceived exertion level is less.

Of course, the other factor is the wind. It's said that the opposite of a headwind is a 'good day.' It's never called a 'tailwind.'