I agree that longer runs are better predictors than 5k's. I was limited to about 45 miles over 4 days per week to avoid injury, so you may be in a better position. The marathon is a multidimensional challenge, so there are lots of levers to play with. Good luck.
I’m not sure if you tried it, but I credit mostly strengthening my hips and core with squats and deadlifts to allow me to increase my mileage. I used to injure my Achilles over 45 mpw before strength training. After strength training I was able to average over 60 mpw with peak weeks in the 90s without trouble. In 2016 I managed 16 marathons including two PRS, a few ultras, and 3300 miles for the year. I was running doubles 3-5 days/week. Just something to consider.
I was fortunate to be able to spend about 7 hours a week on physical exercise when I was racing. My family was very accommodating for this, but my modest physical talents did not justify any more investment. I never felt the pull for Ultras or other endurance events. At 62, I think I still maintain the benefits of fitness with moderate varied exercise, with no training or racing calendar ruling my life. I've been able to scratch several other deep long term itches after I stopped racing.
