I'm always watching speedruns these days. As far as marathons, Chicago Speedathon was this past weekend; they raised $6000 to help get about a dozen awesome runners and back-end people who can't afford the trip themselves to AGDQ 2014, plus a little extra (I don't know the amount) to be donated straight to the Prevent Cancer Foundation at AGDQ itself.
I don't actually run anything myself at this point, but if I were to pick up a game, I'd probably do something I really enjoy that doesn't already have a major competitive scene, like the La-Mulana remake, Monster World IV, or Rondo of Blood (so odd that I've never seen anyone running this, at least on-stream), and maybe do something more popular like a GBA Metroid or Symphony of the Night on the side. I wouldn't be too interested in running most of the mega-popular games, for a few reasons; the big one would be that several of the most popular games are things like Final Fantasy, Pokémon, and the 3D Zelda games, which tend to be anywhere between three and six hours for the categories people take seriously. I can barely bring myself to watch a lot of those games, let alone sit down and play them for that long in a single sitting.