As you read forward, keep in mind that Lamech died only 4 years before the flood and Metushelach died in the same year as the flood.  Meaning that they were both alive for nearly the entire 100 hundred years it took to construct the Ark.

25 And Metushelacḥ lived one hundred and eighty-seven years, and brought forth Lemeḵ.

26 And after he brought forth Lamech, Metushelacḥ lived seven hundred and eighty-two years, and brought forth sons and daughters.

27 So all the days of Metushelacḥ were nine hundred and sixty-nine years, and he died.

28 And Lamech lived one hundred and eighty-two years, and brought forth a son,

1 And after those days my son Methuselah chose a wife for his son Lamech and she became pregnant by him and bore a son.

2 And his body was white like snow, and red like the flower of a rose, and the hair of his head was white like wool. And his eyes were beautiful and when he opened his eyes he made the whole house bright, like the sun so that the whole house was exceptionally bright.

3 And when he was taken from the hand of the midwife he opened his mouth and spoke to the Lord of Righteousness.

4 And his father Lamech was afraid of him, and fled, and went to his father Methuselah.

5 And he said to him: "I have begotten a strange son; he is not like a man but is like the children of the Angels of Heaven, of a different type and not like us. And his eyes are like the rays of the Sun and his face glorious.

6 And it seems to me that he is not sprung from me but from the Angels and I am afraid that something extraordinary may be done on the earth in his days.

7 And now, my father, I am entreating you and petitioning you, to go to our father Enoch, and learn from him the truth, for his dwelling is with the Angels."

8 And when Methuselah heard the words of his son he came to me, at the ends of the Earth, for he had heard that I was there. And he cried out, and I heard his voice and went to him. And I said to him: "Behold I am here my son, for you have come to me."

9 And he answered me, and said: "Because of a great matter I have come to you, and because of a disturbing vision, have I come near.

10 And now hear me, my father, for a child has been born to my son Lamech, whose form and type are not like the type of a man. His colour is whiter than snow, and redder than the flower of the rose, and the hair of his head is whiter than white wool. And his eyes are like the rays of the sun; and he opened his eyes and made the whole house bright.

11 And he was taken from the hand of the midwife, and he opened his mouth, and blessed the Lord of Heaven.

12 And his father Lamech was afraid and fled to me. And he does not believe he is sprung from him but thinks him to be from the Angels of Heaven. And behold, I have come to you, so that you may make known to me the truth."

13 And I, Enoch, answered and said to him: "The Lord will do new things on Earth, and this I have already seen in a vision, and made known to you. For in the generation of my father, Jared, some from the height of Heaven transgressed the word of the Lord.

14 And behold, they commit sin and transgress the law, and have been promiscuous with women, and commit sin with them, and have married some of them, and have begotten children by them.

15 And there will be great destruction over the whole Earth, and there will be a deluge, and there will be great destruction for one year.

16 But this child, who has been born to you, will be left on the Earth, and his three sons will be saved with him. When all the men who are on the Earth die he and his sons will be saved.

17 They will beget on the Earth giants, not of spirit, but of flesh, and there will be great wrath on Earth, and the Earth will be cleansed of all corruption.

18 And now make known to your son Lamech that the one who has been born is truly his son. And call his name Noah, for he will be a remnant for you and he and his sons will be saved from the destruction which is coming on the earth because of all the sin and all the iniquity, which will be committed on the Earth in his days.

19 But after this, there will be yet greater iniquity than that which was committed on the earth before. For I know the mysteries of the Holy Ones, for the Lord showed them to me and made them known to me, and I read them in the Tablets of Heaven.

1 And I saw written on them, that generation upon generation will do wrong, until a generation of righteousness shall arise, and wrongdoing shall be destroyed, and sin shall depart from the earth, and everything good shall come upon it.

2 And now, my son, go, make known to your son Lamech, that this child that has been born, is truly his son, and this is no lie.

3 And when Methuselah had heard the words of his father Chanoch - for he showed him everything which is secret - he returned, having seen him, and called the name of that child Noach; for he will comfort the Earth after all the destruction.

29 and called his name Noacḥ, saying, “This one does comfort us concerning our work and the toil of our hands, because of the ground which יהוה has cursed.”

30 And after he brought forth Noacḥ, Lamech lived five hundred and ninety-five years, and brought forth sons and daughters.

31 So all the days of Lamech were seven hundred and seventy-seven years, and he died.