How far was it along the silk road chang'an to dunhuang to marakanda to antioch to rome?

The distance along the Silk Road from Chang'an (modern Xi'an) to Dunhuang is approximately 1,000 kilometers (620 miles). From Dunhuang to Samarkand (the historical name you mentioned as "Marakanda"), it is around 2,500 kilometers (1,553 miles). Continuing from Samarkand to Antioch, the distance is roughly 1,800 kilometers (1,118 miles), and finally, from Antioch to Rome, it is about 1,800 kilometers (1,118 miles) as well. In total, this journey spans approximately 7,100 kilometers (4,400 miles).