As a young bloke he was racing mostly on the fells. This sort of work is great for your fitness but doesn't develop your flat speed.
Had he spent those younger years focused on track and road he probably would have been at least a minute quicker back then. Also if most of his early racing was on roads around Derbyshire then he probably didn't run many fast courses.
A good example of this is fell legend Ian Holmes. Ian has never broken 30 for 10k or 50 for 10 miles. On the fells even now in his 40s he can thrash 29m guys. I'm sure Holmes has the motor to run low 29s even now. But he would need a couple of years to develop the skill set for the flat running.
Steve Moneghetti was running about as fast at 40 as he was at 22, he had some faster years from mid 20s to mid 30s though.
Jack Foster ran 2.11 in his 40s he was only running 2.12/13 in his 30s.
He came back from an injury break at 50 and off a couple of months of 40 to 50 mpw ran 51.40 for a hilly 10 miler. He then cranked it up to 70 to 80 mpw for 2 months and ran 2.20 for the marathon.