Like you said, it's hard to translate ponies to human form. And there is a TON of variety in body type for athletes, even within the same sport. If you care to research athlete references you can try looking at the USA Olympic team roster: http://www.teamusa.org/Athletes.aspx?page=1
(you can filter by sport - since humans can't fly I would think soccer or swimming or volleyball would be well rounded picks).
If you want my opinion, the "flat chested runner" body type would most usually apply to young runners and serious marathon or Olympic grade athletes. More casual women runners are likely to just look like average women, but with toned/muscular thighs if you can actually see their legs. You certainly don't HAVE TO go with a typical feminine shape with wide hips and skinny waist, but that would still be acceptable for an athlete.