There are many small hikes that start from Freiburg itself--they are not at all wild, but have nice views of the city. The easiest thing to do is just to go up in the hills by Schwabentor--if you make a loop from there, you can finish at the beer garden overlooking the city. But also to the south of the city, like the hiking around Schönberg near the Vauban neighbourhood, can be nice.
For a bit more nature/further from the city, there are nice hikes around Schluchsee in the Black Forest. If you take the train to the Aha station, you can walk up in the hills on the north side of the lake towards the Schluchsee train station. I think that is about 5-6km, so might fit your length requirement well. The scenery is not terribly dramatic, but is quite pretty. This is a quick train ride--maybe 30 minutes from Freiburg main train station?
Another option would be the area around Kaiserstuhl--this is less forest and more wine mountains and fields.
If any of these sound interesting, let me know and I can give you more precise information.