It's called, "Old-Man-Calf-Syndrome"!
A chiropractor friend of mine thinks that it may be because of nerve compression in the spine. You don't even know that it's there - until you start running and then, after 10 - 15 . . . wham! Your calf completely locks up!
Its definitely an issue I have and others in my industry: sedentary desk-bound dronies. I'm 49.
Since there are a lot of people, in middle age, that have management type jobs, it might be enough to affect a statistical average.
Back to the issue...
Sitting on your a$$ all day creates a weak core, and especially back, core and gluteal muscles.
Spinal compression happens, the nerves exiting L5 - L3, (the ones driving lower leg muscles) are affected.
Poor conductivity results in poorer muscle control and strength.
Sciatica is a common symptom - but IMO covers a lot of different issues.
I've had this on and off for about 10 years.
Training for an ironman has hugely increased my core strength, reduced the symptoms, and improved my ability to handle every day situations. 3 years ago I could hardly walk to the car park about 500m away - had to sit and stretch just to keep going!!!
So its a huge struggle for distance running.
Lack of power and control in my affected calf is the biggest issue.
Standing on one leg and doing some yoga moves really highlights such issues. My affected leg can't keep me stable. its not just the calf, but all the other muscles as well
Interestingly, I've noticed that my parents and friends, all in the 70's - 80's are frequently unstable on stairs and other challenges. When watching them, they seem to have really poor power in their lower leg muscles.
IMO muscle imbalances are the issue I can fix.
- My erector spinea muscles are imbalanced. Strengthening one side and not the other is tough - our bodies ability to compensate is amazing. its some to check
- I get "cramps" in the muscles at the base of my spine. Lack of mobility in the L5-L3 area. Electro acupuncture has been a great short term solution. But only lasts about 2 weeks. Chiropractic has been a waste of time.
- 5 Finger type of "shoes" might actually help! Well... for training, but I wouldn't race in them.
As I've gotten stronger, my distances have increased, but anything over 15k is tough.
I'll crawling across the finish line in Cozumel....sigh.
If any of this sound reasonable to you, please let me know. Maybe my experiences can help.