If I was absolutely sure of being the last person on earth, I would do some research, drive to the nearest pharmacy, and take whatever drug would be the most peaceful to die of an overdose from.
I think the "last person on earth" thing would be a lot like Sartre's
No Exit except without the other people. I'd have massive survivors guilt, and I'd be unbearably lonely. I would become my own torture device, and would go insane if I kept living after a year. Perhaps I would take the first year after the disease to see how long I could survive while reading and exploring the world, eating whatever has been left over from before. Then after, I would off myself.
I wouldn't bother repopulating the earth with people. I figure that if the entire human race is wiped out, we must be making room for something better to come along. (maybe I'd read and sleep and wank until this something better showed up...
)