It would not be taxed at a flat 20%.
Because you've held it for over a year, it's considered a long-term capital gain. The amount of the gain using your numbers is $85K - 65K - depreciation. Depreciation is just on the building, not the land. Look at your old tax returns to figure out how much you depreciated it during the time you've owned it.
The happy news is that if you are married filing jointly you are not even close to the point where you'd have to pay federal income tax on this gain. You would have to pay depreciation recapture. But given that you have not owned the property long, that should not be a huge amount. Also you may owe state tax.
Congrats on making a good investment!