KenB123
I recall Lasse Virén as one great distance runner from Finland (4-gold: 1972, 1976).

Those were the days! Mo Farah now stepped beside him winning both 5000 and 10000 metres in two
subsequent Olympics. The funny coincidence is that on 10 000 meters they both fell down, but got up and won. Lasse Virén did a world record still.
In Montreal, 18 hours after winning the 5000m, he ran his first marathon the next day, and finished fifth.
So within a bit more than a week he ran both 5000 m and 10 000m qualifications, finals and marathon.
 
We must remember/ as you know, though, that the first versions of blood doping were very popular in early 70's, and there were no ways to spot it, and actually, it wasn't even forbidden first. Athletes put their own blood in storage and took the extra fuel back before the games. The flat-country explanation was it just compensated for the advantages of training in the mountains.
Of course, I'm not saying the long distance runners, not to mention some others, don't have their ointments and little helpers today.