Shooting down a Mach 6 aircraft is extremely difficult.
Lets say an SR-72 was going to go the full length of Iran, and Iran had recently deployed S-300 missiles from Russia. The S-300 is considered a world-class air defense weapon (despite having never been fired in combat). It has a 5 minute deployment time and a 24 mile range.
Mach 6 is roughly 4,567 Miles/hour or 1.26 miles every second.
It will cover the 48 mile engagement envelope of an S-300 (24 miles each way), in 38 seconds. What this means is a missile site can't detect and engage the target. Someone has to detect and transmit targeting information to air defense sights in the path of the plane, so they can be ready to lauch, when it gets within range.
Just some moderate maneuvering and route planning, keeps the SR-72 out of range most of the time.
There was rumor that the SR-71 was detectable with long range radars, but stealthy to weapons guidance radars. Add in stealth characteristics and the task becomes even more difficult.
From looking at a map, the absolute longest flight path over Iran appears to be about 2000 miles. Meaning the SR-72, worst case, would only be over Iranian airspace for less than 30 minutes. If a plane came in over the Caspian Sea, crossed over Tehran, then turned for the nearest border, they could be in and out of Iran in less than 5 minutes.
All in all, a very challenging exercise.