That's a fairly common number. SLEP usually happens around 5K, with an extension to ~8-10K based on periodic inspections. The Block 0 and Block .75 Bravos were extended out to 12K if they still had life in them and anyone wanted them. By the late 2000's as Romeo started coming online, there were a surplus of Block .75s so the only place that still flew the Block 0s was -60 or Pax.
Interestingly, the Block 1s were the ones that were at end-of-life earlier because they were deployed more since they were shooters.
The Block 0 that I took to the Boneyard was in the ~11K range. The airframe was fine, but the only thing on the mission systems that worked was the Tape Handling Package and the MPD.
It would be interesting to know where the weak points are on a -53E. For the -60s, it's the drag beams and the Tranny support beam (my words...I forget the actual thing it was called).