That was definitely my experience when I started hitting my PRs during endurance training. Obviously the running component his huge, but adding in the resistance training was a huge part of my improvement. Not to mention the decrease in recurring injuries that popped up from running high mileage. Apropos to "should" do, I recently read an article on how certain aspects of training theory is "path dependent", that is still being utilized based off old science and has not changed with updated knowledge gained from current research. The article was on how to apply periodization to training, but I think it can apply to how endurance athletes utlize strength training. Historically we'd see recommendations of high rep sets and low weight to improve muscular endurance. While its true that high rep sets will produce adaptations specific to muscular endurance, this is moreso in specific mvoements and ranges of motion. And when looking at performance outcomes, which is what I would argue really matters, we see that lower rep ranges tends to produce better outcomes in endurance sport. Heres the article on path dependency https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5856877/