Why it seem I can pay off most mortgage in less than 10 yr?

[Orlando Goodon]

It seems to good to be true. My math must be wrong. Please let me know what I'm missing. Basically let's assume no depreciation and not exceeding $300-$400/month maintenance budget to much.

House price $750k for 3 family with 2-3 br on each floor in area with $2500-$3200/mo rent. OK:

$30k down

$720k financed

$4500/mo Mortgage with tax, insurance and PMI

$8500 Rental income + $1000 (additional payments to principal)

7 years (Time to make 85 payments of 8500 which equal the financed amount)

What am I missing? 8500/mo for 7 years is $720k-ish. No need for 30 years to pay this off right? Am I mistaken? I've done the math on several properties and get same results.