This summer I spent 4 days in Rome in June. As with vacations in general, I did quite a lot of walking. At times it seemed like more than just "a lot", so I traced my walking on a map so that I could figure out how much "a lot" really is. It turns out that you can cover pretty much all the sights in Rome on foot, they're not that far apart. A walk would take anything from 1.5h to 2.5h. It turns out I would cover up to about 13km in that time. In three and a half days (also took a half day trip to Ostia Antica) I did about 40km of walking.