Blowing out moisture from gas line to furnace

Thanks. Most of what you said is way over my head. Unfortunately, the "family" friend HVAC guy is a crackhead and nobody wants him around. I've not opened the drain in 5 yrs. I did blowout the 1/2" black pipe before I hooked everything up and all kinds of rusty blackish water came out. The underground black pipe sat for 5 months before I hooked up the furnace. The furnace has always kicked on, it just takes several hours. Would it take the low pressure gas from the house hours to get to the detached garage 75feet away? Thx