Unless someone invents a time machine, we will probably never know how strong our ancestors were. I don't have a strong opinion one way or the other, but here is a plausible mechanism of how people could have been stronger in the past:
1 - Desired characteristic (strength) depends on many genes. The more you have the "good versions" ("strong" allele for those who want the scientific term), the stronger you are.
2 - Selection pressures make weak people die more often in a certain region (think of Rippetoe here)
3 - Due to lower mobility than today, the strong people (or at least those with the "strong" alleles) mate with each other in that region.
4 - Once in a while, an individual is born with a lot more of the "good versions" of the genes, just by random combinations.
5 - Not required but possible, there was also a chance mutation in the population that gave a great advantage in strength.
Today, having as many "strong" alleles will be more difficult as there is less selection pressure and more mobility, so strong individuals are dispersed over a larger area.
Also, in genetics, there are recessive alleles that can increase this effect. We all have 2 versions of most of our genes: one version from our mother, one from our father. In a recessive alleles, the characteristic is expressed only if both versions are the same. Therefore, your father could have two different versions of gene A and your mother the same, and not show the characteristic and by chance you get the recessive version from both for gene A. There are genetic diseases that work like this. The parents are healthy, but the baby has the disease because he/she was unlucky enough to get the recessive version from both parents, and that version is defectuous). So, if strength is given by a recessive characteristic, it will be much rarer than if given by a dominant (the opposite of recessive) characteristic.
I'm not saying that this is what happened, mostly because as far as I understand, these mechanisms would only work over many generations, but it's at least plausible. In fact, isolate the people enough for long enough and you get eventually a new species. By the way, some genetic diseases are much more prevalent in some regions of the world because of the limited mobility of the population in the past (informally referred to as "inbreeding"), so this mechanism had been proven to work for some types of recessive alleles.
[EDIT]
For an example in endurance sports, see
Eero Mäntyranta. Not eactly the same, but close.