There's a longer article about Hagg, and Swedish milers, in a book about milers, which might be the one you are looking for. I have it in a box somewhere. The "How You Train" book doesn't have as much.
How They Train, by Fred Wilt, 1959 T&FN.
I think Gunder Hagg's father got him started with running, the same method he continued to use.
Born 12/31/19, 6' 150 lbs.
Started racing in 1936 at age 17. Stopped racing at age 26.
Hagg designed a 5000 meter forest path and ran over it daily, from December 1939 through spring, 1940. Often in winter he had to fight with both arms and legs through huge snowdrifts to complete the course, frequently in temperatures much blow freezing. These were continuous runs and he was extremely fatigued after each. A considerable amount of skiing (and also military marching) was also included in his training.
This particular 5000 meter loop went as follows:
start - uphill - level - downhill - level ground full speed 800m - uphill - downhill relaxed running - level area along river, full speed striding mixed with sprints - uphill even speed to finish