It depends a lot on the terrain (uphill vs. downhill) and whether you are walking fairly fast or taking your time. Generally, when hiking, a long full day of hiking is something like 8 miles... I think that you could do 12 km in something like 5 or 6 hrs. It's difficult to say though. It also depends how many breaks you take and how long your breaks are. I would give yourself the day though, and you can take your time and you'll be fine.
