I didn't make it!
Well, to somewhat answer your question, the seven hour war most likeley involved far more aggressive and high tech combine weapons to combat the US military. Naturally once the bulk of the human military is wiped out and you have police states like City 17, there is not a need for such high-end equipment, and lower grade weapons and soldiers are used to police the area. You have to look at the lost cities in HL2, you have scattered rebel cells from the humans, with limited conventional weapons, and tactics, and the rest are largely non combatant civilians just trying to live under the Combine's thumb. I'm sure that all military systems have the bugetary beauracracy that plagues our military today, Combine being no exception, and they simply are using the equipment that would be required for the threat level that the cities pose. I'm in the military, and thats my slant on the whole situation from experience. It's simply cheaper for the combine to use overwatch and the headcrabs than it would be to have maximum firepower over the city. You also have to take note that the combine has shield technology, as well as the suppression fields, which look more like orbital energy beam strikes. As tough as the rolled homogenous armor is on most military fighting vehicles, I think that anyone would have a tough time with combine technology. Just take a look at War of the Worlds and you'll have an idea of where I'm coming from. As for 7 hours, its not too unrealistic if the combine could orchestrate a planetary strike via suppression field technology. Why bother with a ground war when you can simply pound the earth from space. Its the mentality that we had in the first gulf war, with the first 100 hours being an air campaign, just on a larger scale. Anyway, I hope that answers some questions for you curious types who are skeptical about the future which Gordon freeman finds himself in.
Just as a mention, you can't judge the combines military power based on its "peacetime" forces. When you're in the citadel, you can see row upon row upon row of stored striders, gunships, etc. Considering this is not the only citadel, it is fair to say, they actually used all their hardware during the attack. The earth military would have already been worn down, theres just no way they could have defeated it all.