You need to throw at least 87 MPH at optimum angle at sea level to throw 300 ft. 86 MPH at optimum angle at 1000 altitude to throw 300 ft. Not everyone who throws "80's" can throw 300 ft.
There is not necessarily a correlation between pitching hard and throwing for distance. Bum, Jr. threw long-toss for three obstensible purposes, all of which augmented his pitching ability but did not guarantee it (only pitching instruction, stuff and experience can do that):
1) Improve arm speed
2) Reinforce arm slot
3) Promote arm health
Don't confuse long-toss with pitching. There are long-tossers that can't pitch, pitchers that can't long-toss, and pitchers that can long-toss.