I recall putting some similar math to it back when I saw someone on another forum crowing about getting to 60,000 miles in 27 years. That averages to 2222 miles/year and (allowing a couple of weeks off each year) 44 miles/week and (allowing one day off each week) 7.4 miles/day. Also generously allowing an average pace of 10 min/mile, comes out to 10,000 hours. Yes, 27 years to get to 10,000 hours. In all that time, a marathon best of 3:20+ (for a sub-masters male) resulted — no surprise. This points to the fact that, at least for physical pursuits (maybe less so for disciplines such as writing, music, etc.), the concentration of those hours may also play a significant role. I could see someone with decent talent for distance running averaging something like 2500 miles/year for their first 4-6 years – through high school and perhaps the first couple years of college – yet after that it should start ramping-up significantly to 4000+ miles/year. A runner such as that would get to 60,000 miles in 10+ fewer years than the example at the start of this post. A PR about 40-60 minutes faster would come as no surprise, either. A seven min/mile average over a career like that would bring 10,000 hours in 19-22 years, so if starting at age 14 then a peak at age 33-36. An eight min/mile average over a career like that would bring 10,000 hours in 15-17 years, so if starting at age 14 then a peak at age 29-31. Plus, as you noted, time spent doing form work, core work, etc. could add at least two hours/week – or 100 hours/year – to that (on the same token, spending hours each week jabbering on the computer about topics ancillary to running would likely impart a negligible effect 😉 ). Adding that in would bring those ranges down to 16-19 years (ages 30-33) for the seven min/mile average and down to 14-17 years (ages 28-31) for the eight min/mile average. So one can see why Africans can dominate as Junior and Senior athletes at young ages if they start at age six or so – 4-8 years before most of the best U.S. runners – even with a more gradual progression.