Impossible is just a big word thrown around by small men who find it easier to live in the world they’ve been given then to explore the power they have to change it. Impossible is nothing.
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.
