Running a marathon quickest requires stamina and speed. There's nothing to suggest Calzaghe could run fast.
He was lean, didn't have a lot of muscle which uses far more energy. This enabled him to maintain a fast pace for the full duration of a fight.
The quickest no, but he would be more likely to finish a marathon than most. (Yes, there are many elite and world champion boxers who probably couldn't last...not every pro athlete is suited to every type of athletic event... you wouldn't expect a linebacker to have very good handles on the basketball court, for instance...or expect that a javelin thrower would automatically be a great swimmer...)
I liked Calzaghe but for a retired fighter, there seems so many threads a month about him. Still in answer - Some boxers just have fast hands and can throw fast combinations. Calzaghe like many others trained to improve and use speed and fast combinations
He was on tv once in an interview, think it may of been uk sky sports ringside special, where he said due to bad hands he had to change his style, earlier in his career he was more of a banger and knocking people out, and it caused bad hands, so he adopted a more speed and less power approach, hence the pitter patter style. Stick it in youtube its on there.