Like lastplace, I used to do a lot of running and only gave up competitive running when I was 34 due to knee problems. 10 years later and I was going bananas- the knees were finished- could not do ANY leg exercise but took up Cycling. 15 years later and still riding and doing some quite lengthy strenuous rides, occasionally.
As to milage- A marathon can be done in around 4 hours at our age. A 50 mile bike ride in about the same time--(Talking average now) So double your running milage and keep pedalling.
Only thing is- A marathon will kill me- 50 miles on a bike is a mornings jaunt with coffee and buns thrown in.
__________________
How long was I in the army? Five foot seven.
Spike Milligan