its kinda old news.... though i admit i only have the anime to go on pre-shippuden, im fairly certain its revealed during the scuffle between Sasuke and Lee before the Chunin Exams... it was in the anime at least
I think the turtle represents the tortoise from the tortoise and the hare. Basically saying Guy's consistently strong training has helped him defeat many "hares" who were naturally gifted and strong from youth. Even the fact that Guy's a match for Kakashi and that Itachi warned Kisame not to underestimate Guy shows how an untalented kid worked his way up until he became a great ninja.
That's why Guy loves Lee like his own son, because they're the same basically.