Iran is three times larger and not very flat. About 270,000 troops were needed to invade Iraq. (See: Military preparations for 2003 invasion of Iraq). You would need about 800,000 additional troops to be gathered, supplied and positioned on Iran's borders.
This step only would take months and huge financial/political cost.
Would agree that given the current situation, we're not going in. However, you can't discount the possibility that we might need to go in given Iran's support for terrorism and efforts to get the bomb, and if we did not have operations in Iraq and Afghanistan, it would be a whole lot bloodier and more expensive.
To continue the analogy with the former USSR, the Cold War wasn't an effort in preparing to invade the Soviet Union either. The idea is to keep up the pressure and provide support to internal elements in order tip the balance toward some sort of popular revolution (hopefully a relatively peaceful one as did occur in the USSR when the Communists attempted a coup and were crushed by popular and military opposition).
The strategic flanking of Iran on all sides is similar to what we did to surround, isolate, contain, and pressurize the Soviet Union. It's a gamble but the alternative would be to let things stumble on toward armageddon.