Milton is contrasting the seeming speed of life as it flies by and is finished to the seeming long length of individual days. He is also contrasting the ultimate end of human life with the ultimate annihilation of Time.

Fly envious Time, till thou run out thy race,
Call on the lazy leaden-stepping hours,
Whose speed is but the heavy Plummets pace;

The first line quoted here carries a Biblical allusion to the Christian notion that Time will eventually end, "till thou run out thy race," when Earth, space and time cease to exist at the end of the world, a time when a New Heaven and a New Earth without the limits of Time is expected. He defies Time's control over individual human lives in the words "Fly ... Time." The poetic narrator (presumably Milton himself) is mocking Time for two reasons. First, though life rushes past, each day has a "lazy leaden-" pace that goes only as fast as a lead weight, "Plummet." Second, Time will cease to be, "thy greedy self consum'd," but humans' lives will end in the long eternal "bliss" of unity with God:

And last of all, thy greedy self consum'd,
Then long Eternity shall greet our bliss
With an individual kiss;
And Joy shall overtake us as a flood,

In summary, Milton has developed a wonderful metaphoric paradox for understanding Time as something that is both too fast and yet prolonged: the whole is too fast but the individual parts are preciously slow. Milton further "pitted the flight of Time against the 'lazy leaden-stepping hours' and 'the heavy Plummets pace'" as a mark of defiance against the quickly spent short course of life and as a celebration of the eternal blissful life that is to follow.