what differences would you see in the heart of a world class distance athlete?

Answer 1

This is not a matter of size or morphology, rather athletes have improved stroke volumes, and thus improved cardiac outputs compared to normal individuals. This also means that they can sustain normal homoeostasis and metabolic activity during rest at lower heart rates than normal individuals. Normal adults have resting heart rates over 70 beats/min, athletes normally below 70 and closer to 60 even.

Answer 2

None, everyone's heart is about the same size.