The only thing you really have to do on the 20 hour is change the oil and filter and the lower gearcase lube. They preload the engine with a heavy weight oil, so you will notice a difference in RPMs when you change it to the recommended 10w-30. The rest is really just a look over making sure everything is tight. Though I believe they do say to check the valve clearences I believe on your engine (90 hp), on my 150Hp I dont have to do that until 200 hours.
You can find a copy of your manual here
http://www.honda-marine.com/owners/OwnerManuals.aspx
As for going over in hours, I doubt seriously if you would hurt the engine by going to 25-30.