If you took all of the blood vessels out of an average child, and laid them out in one line, the line would be over 60,000 miles long and an adult's vessels would be closer to 100,000 miles long.
If they could be laid end to end, the blood vessels would cover a distance of about 60,000 miles (96,500 kilometers).
Sources: Vogel, Steven. Vital Circuits, pp. 15-16; World Book Encyclopedia, vol. 2, p. 424.