As somebody with a really bad back, I'll add perspective.
#1 : As an older guy (your blog puts you at 51yo) your back has gone through a lot. There is a chance that the spine has degraded -- through the natural loss of bone mass, arthritis, stenosis, degenerative diseases (
Spondylosis) and the aggravations of age on congenital defects. Therefore before you get into the exercise of the day, it pays to have your spine scanned. X-Ray, CAT, MRI are the scans shown in order of increased cost and clarity.
I was given exercises for years with limited results. Then I walked into the office of a sports orthopedic surgeon who took one look at me and ordered a scan. I was born with 6 defects in my lumbar region. I had an extra lumbar vertebrae. All of the lumbar vertebrae were small, which created two problems: the inside of vertebrae had narrowed with aging causing a stenosis that was pinching my spinal cord and weakening leg strength, and the small size of the bones caused massive compression on the discs when I was squatting crazy-heavy weight. Eventually I blew two discs, had surgery, and the back hasn't been the same since.
And just because I had that surgery, it doesn't mean the back quit aging. Recent CAT scans have revealed s
pondylosis, arthritis and foraminal stenosis. It was only after my most recent scan that my current sports ortho doc gave me exercises. At age 50 and beyond a scan is helpful.
One more thing. I just noticed this year, and it goes back to the dream time and my previous racing: spend more time in little gears early in the season. Back in the day the maxim was 1,000 miles before you ever pedaled bigger than a 70" gear, I'd get this lower back pain at around 20 miles. By about the 1,000 mile mark that pain would go away. And that was right about the time I started pushing the bigger gears. Fast forward to my re-entry to the sport and for the past two years my cancer had me forgoing that base work and I was having big back problems, including debilitating spasms. Because of COVID this year is a training year and I'm kicking it old school. Sure enough, that 20-mile pain came back. BUT, it was low stress and lower, early-season miles. Like in the article you posted, it was low-stress repetition and rest that allowed me to strengthen my back. Now, I'm just starting to push the bigger gears and I'm doing it with a lot less pain than the previous couple of years.
So, for me, lower gears early in the season, and higher-rep/lower-weight squats in the weight room. And since you have hit 50, get that back scanned.