A few chapters later (and now this is indisputably history not allegory), nobody lives to be 900 years old. But the Avot do live significantly longer than anyone today - up to age 180. And many events in their lives take place when they are much older than would seem normal. But imagine that the aging process of the Avot was slowed down by a factor of two, just as the aging process of their ancestors had been slowed down by a larger factor. Let's recalculate various events in "avot years", these being equal to exactly two normal years, and see how much of the chronology suddenly makes sense.
- Avraham was 37 (75) when he made aliyah, young enough to walk several hundred miles.
- Sarah was between 32 (65) and 45 (90) when Pharoah and Avimelech found her extremely beautiful.
- Avraham was 50 (100) when Yitzchak was born. More importantly, Sarah was 45 (90), just past menopause, as she hints in her comments. If you object that birth at this age is not miraculous enough, remember that to outsiders the couple would simply have appeared as very healthy nonagenarians.
- Yishmael was 6 (13) when he was kicked out of the house (Breishit 21) in a story which makes him seem dependent on his mother.
- Avraham died at 87 (175), Sarah at 63 (127), Yishmael at 68 (137), Yitzchak at 90 (180), Yaakov at 73 (147).
- Yitzchak married at age 20 (40) and had kids at age 30 (60). Esav married at 20 (40) too.
- Yaakov and Esav were 31 (63) years old when Yaakov took the blessing and started the feud between them.
In the next generation, Yosef led his brothers at age 17 and died at age 110. The ages of his siblings are not clear, but there is already a problem of them being too young in the Dinah story (see here) and so it is hard to argue that they were physically even younger than their ages. I would therefore argue that Yaakov was the last to have an extended lifetime. Starting with Yaakov's kids, everyone aged at the rate we expect today.