I was making mixes/playlists the other day. I don't make mixes very often because it's an agonizing process for me. Seriously. I started with over 70 songs that I had to dwindle down to 31 to fit on two CDs. It was torture.
See, for me, the songs have to flow. I can't just throw a completely random bunch together - that's a different process. No. The end of the song has to flow well enough into the beginning of the next song. I don't like having two songs in the same key back-to-back. If I include the same artist more than once (Bruce Springsteen, Bob Seger, the Rolling Stones, the Who, Led Zeppelin, Van Halen, and Tom Petty all make multiple appearances) then I have to properly space them out. It's no easy task.
That being said, I still want to be a Classic Vinyl DJ for a day. It might kill me, but it'd be worth it.