If they are to invade, thats how they will do it
Deploying 120 000 troops takes lot of time. Iran can attackfirst then.
Three main targets will be attacked in the opening days with ballistic missiles.
First, US bases and ships.
Second, gulf oil production.
Third, tankers and pipelines.
If US manage to invade, 120 000 troops won't be enough at all. They will always be in range of scud d type missiles just like in iraq wars but iraqis had very bad missiles. Iran has thousand of kornet type atgm which will be as good as houti's ones against abrams and other humvee and they are 70 million people for guerilla war.
They also have manpads to keep away air support like a-10 and choppers. Their air defences won't last long enough to be an issue however.
IMO, Iranian should be very mobile and very well equiped with atgm's, manpads and anti armor snipers. GPS jammers for everyone. Lasers pointed on the ground around major infrastructure in the hope to make laser guided bombs follow the wrong laser. Huge barrages of BM on oil field and US bases will destroy some economies.
Its navy should be used the first day quickly and in big numbers to achieve at least one carrier destroyed and its subs could destroy some ships too. All the tankers in the persian gulf will be destroyed.