Just an anecdotal observation, people I know that run 70+ miles a week, and have full time jobs/families, etc. have to get up at 4:30 AM to get the miles in or

Just an anecdotal observation, people I know that run 70+ miles a week, and have full time jobs/families, etc. have to get up at 4:30 AM to get the miles in or rearrange schedules to fit everything in. So maybe any health decline is due to a decrease in quality sleep and increased stress to get the miles in, not the increased mileage. If you have tons of free time, run as much as you like.