Another interpretation: "Go forth" [lekh lekha] at a hundred years you will have a son, the numerical value of "lekh lekha" [the letters of the two words lekh lekha total one hundred].
The Numbers In Go Forth Promised A Son At A Hundred
Yalkut Shimoni on Torah 63:1
דָּבָר אַחֵר: לֶךְ לְךָ, לְמֵאָה שָׁנָה יִהְיֶה לְךָ בֵּן, מִנְיַן 'לֶךְ לְךָ'.