Why does the Torah mention the total number of years of each of those listed before the Flood, although it has already told us their age before they gave birth and how long they lived afterwards, something that it does not do for those who lived after the Flood (at the end of No'ach)?
Ramban: Because, compared to all those who lived after the Flood, they lived for so long.
Why indeed, did they live so long?
Ramban: They lived so long because Adam ha'Rishon was created by Hashem's Hands, and was therefore the perfect specimen - as regards strength, looks and stature. Following his sin, despite his death-sentence, he still had the ability to live many, many years.
What happened to curtail man's lifespan after the Flood?
Ramban: It was only after the Flood, which adversely affected the air, that man's lifespan began to gradually shorten - to around the four hundred mark.
And what happened to curtail their lifespan again after the Generation of the dispersion?
Ramban: The same happened following the Generation of the dispersion, when the air was once again affected, and their lifespan was again curtailed by a half. 1 It continued to diminish until the era of the Avos, by which time the average lifespan settled at seventy or eighty. 2
Like we find by Peleg, in whose time the dispersal took place, and who lived two hundred [and 49] years. (The Torah lists nine generations born after the flood and before the dispersion. They lived a total of 438, 433, 464, 249, 239, 230, 148, 205, and 175 years. One could say that Peleg died in the year of the dispersion due to the air - but Nachor (Terach's father) died at 148 before the dispersion! Each of the middle seven was born 29-35 years after his father. If the decrease was due solely to the air, why did lifespan drop so suddenly? It should have decreased gradually! (PF)
As Moshe Rabeinu (cited in Tehilim 90:10), attested. The Avos lived much longer, because, as the Pasuk says in Mishlei "The fear of Hashem lengthens days." (We also find Resha'im who lived long - Og, Paro (one opinion equates Paro in the days of Moshe with Paro from the days of Yosef), Nebuchadnetzar (he lived over 150 years after he was in Sancheriv's army in the days of Chizkiyah, 130 years before the Churban)...!) (PF)