I'm only 5' 7" and always tucked in around 10 stone for most my life.
My running only really started when my hamstring was a reoccurring problem for me in the mid 2000's.
My first marathon was in 2007 around 43 years old.
I was never a fast runner in running but was regarded as fast when playing football. Very fast over fifty meters and would cover about 12 kilometers during 90 minutes of football.
My best 10K was probably around 46 mins....half marathon about 100 minutes...best full marathon was 3:45 when I was about 52 years old.
Now if I run fast my ACL will scream out. So I keep to a certain pace which is never flat. Always with hills as this keeps me working different muscles sets.
I know you were a fast runner in your day.