Dear Freckles,
Aragorn is a descendant of the first Numenorean king, Elros (Elrond's brother, and one who did not sit around doing experiments to extend his lifespan). The last king, Ar-Pharazon, was a royal stinker who forced his cousin, the rightful queen, to marry him, and then proceeded to sacrifice humans to Morgoth. He eventually decided that running experiments on the eternal life thing wasn't working very well, so he tried to sail to Valinor, thinking that once he reached it, he would achieve eternal life within the world, like the Elves. Only it doesn't work that way. He was killed after landing, and Iluvatar (God) then sank Numenor in the sea and made the world round so that only Elves could sail the "straight road" into the West. Aragorn's ancestors (nobility by then, not kings) were expecting something like this to happen and had ships ready, so they managed to sail away from Numenor before it sank. They made it to Middle Earth and started the kingdoms of Gondor and Arnor there, and fought Sauron in the War of the Last Alliance. Funny thing about the Silmarillion...when you find out more about Isildur, you realize he was mostly a hero, except for a certain unfortunate incident with a Ring....
Hating the Round World,
Glen