Where you posted is just fine. The nasty stuff you have pics of is just that. Get a duct cleaning company to come out and give you some prices, My advice is to stay away from the ones who blow it out with air or vacuums. To thoroughly clean ductwork it needs a good scrubbing. This means cutting and then patching holes into the ducts for access. Also get your unit serviced at this time, check your humidifier if you have one for more nasty stuff.
Let us know what they give you for advice, the only advice you will get here is "honest advice".