Running

Yeah, got 2 marathons this weekend as well - how many ibuprofen constitutes a just-below-fatal dose?

Ibuprofen: ORAL (LD50): Acute: 636 mg/kg [Rat]. 740 mg/kg [Mouse]. 495 mg/kg [Guinea pig]

29.7g (148 x 200mg) for a 60kg Guinea Pig, 44.400kg (222 x 200mg) for a 60kg Mouse

Are you more Guinea Pig or more Mouse?

Those would be doses for a 50% chance of dying (or 50% chance of living if your glass is measured that way).

I wouldn't take more than it says on the back of the packet.

how many ibuprofen constitutes a just-below-fatal dose?

The simplest way to find out is keep taking them until you die, then the answer is one less than that. I'd go with tramadol in a more reasonable amount and trying not to die even once though.