According to the earlier posts (which you seem not to read, instead posting the same question dozens of times - which is wearing a bit thin already) the firing range was nearer 73km, and the 80km lock range was not updated after jamming commenced.

The missile has a 34km flight/interception range, the target closes the remaining 40km in the flight time of the missile, which incidentally confirms intercept to be impossible against a receding SR71.

The missile could be fired at closer ranges, but then the crossing angle is higher and the missile must turn more to track the target - this is likely to exceed guidance/manoeuvre limits in a steepening intercept geometry - using the longest practical range greatly simplifies intercept geometry.