Until it did. TYG.
I didn't like the urban dictionary definition of TYG, so maybe you can explain that.
As for the story, it didn't work until the grace came. That's why they were "old" - to point out that natural sex didn't work with them for a long time. SO: the ONLY addition to their efforts was the blessing. THEREFORE: the conception was through divine grace. Why is that hard to understand? What other possible meaning is there to the story of an angel showing up and giving a conception blessing, and then them conceiving?