i chose D2 wrote:
So who was it?
Well going off the marathon world record progressions, the first runner to go under that 2:06:36 threshold was Ronaldo da Costa in '1998.
But other posters are correct that, as athletes got nearer to that 2:00 hour threshold, the motivation would've been even greater. In '88 Belayneh Dinsamo ran 2:06:50, which is close enough that I think he could've cracked it if he were chasing it.
Carlos Lopes was also close, running 2:07:12 in '85 but that was near the end of his career, so it's hard to say if he'd have had enough in him at that point to drop another thirty seconds.
I don't think DeCastella could've done it, as his PR (2:07:51 at Boston '86) was more than a minute slower, and right after he ran that time, he never came close to it, nor did he win another major marathon.
So the long short of it is, da Costa was definitely the first to break 2 hours, but Dinsamo definitely could've done it as early as '88 had he been chasing it.
The big unknown of course is, could that 2 hour barrier have motivated some Athlete X to chase it, who without it never really challenged himself. There is always the chance someone might have emerged who otherwise wouldn't have.