I have lots of experience, and I know the feeling of being a longterm runner in his early- to mid-fifties who is "somewhat freaked out" by glomerular filtration rate (GFR) measurements indicating stage 3 chronic kidney disease.
I can't spend much time on this answer right now, but I want to point out that, in all likelihood, your GFR numbers were probably obtained by a fairly simple test that measures creatinine concentrations in the blood. It's a useful test, cheap and easy, but it provides only an estimate of GFR ("eGFR") that can be affected by many factors, including dehydration, muscle mass, and physical activity. The gold standard for measuring GFR uses an exogenous filtration marker to directly measure rate of clearance through blood or urine over a period of hours, and that test may provide very different (and more accurate) figures for your glomerular filtration rate. At some point, it may be worthwhile to arrange for such a test, especially if your eGFR measures are showing a fairly rapid decline.
If you're at all like me, you'll start reading everything you can get your hands on. But don't get too carried away by any particular articles or studies; it's a big subject. You may find it useful to check out what some of the more reputable medical facilities (such as the Mayo Clinic) or organizations (such as the National Kidney Foundation) have to say.