I'm 48 and after a long time off running started again last year, training for a first marathon in December. I did a 16 week plan that peaked at 70 miles, running everyday and had no doubles in it. My December race was cancelled and then I ran a 2:52 virtually.
Given this was not official, I had a week off, got a real coach and then started a new shortened plan for a target race in March. This plan peaked at 80mpw and included 1 double run day, both runs being easy efforts, with normally 7-8 in the morning and 4-5 in the afternoon. This definitely allowed some extra mileage volume with less stress on the body. This plan also added an extra workout during the week so I went from one workout and a long run each week to 2 workouts and a long run. It was tough and I think I balanced on the edge of over-training and possible injury but I ended up running a 2:48 at the race and probably had more to give as ran 6:15's for the last 6 miles to negative split (helped by being dragged along by a faster runner)