A nice tour starting from Gersthof (end of Kreuzgasse), going around Simonsberg and Hermannskogel.
The way up Czartoryskigasse is a nice warmup, you should arrive up there with a good heartrate and warm legs.
Many variants exist between the top of Schafberg and the parking lot at the end of Poetzleinsdorfer Strasse, with various levels of steepness, muddiness and rocks/logs. You can easily take a shortcut along the fence of Poetzleinsdorfer Shlosspark.
The way up through the woods between Keylwerthgasse and Hoehenstrasse is particularly enjoyable and not too exhausting. It is a long ascent though, so make sure you don't burn all your energy at the beginning.
The little loop around Simonsberg is optional and offers nice views over Lower Austria. It is a very quiet path, on a well maintained forest road.
The path down to Sieveringer Strasse is not well indicated. The map seems to indicate that it starts shortly before the hair pin crossing at the end of Simonsberg's loop, but it actually starts shortly after it. Make sure you look around and find a small path going right of a small house down into the valley. This part can be quite mucky in winter, with shoots of trees growing in the middle of the path.
Getting off Hauptstrasse to the right after the small house, you will see a pole with yellow signs indicating directions. The way up on the flank of Sauberg is one of the steepest bits of this trail. You might have to carry your bike on your back for a few hundred meters, especially in wet/muddy conditions.
Halfway through the straight line heading away from Hauptstrasse you will cross a forest road. Make sure you follow the road to the left, or you will keep going up-hill for quite a while (leading to a variant of this trail that takes you up Sauberg and down again to Hermanskogel).
The way up Grosser-Frauengraben is very steep and you might also have to carry the bike on your back for a while. A few variants exist that might be a bit longer but not as steep.
The way down from Gasthaus zum Agnesbruennl is a bliss, with wide and varied paths, lots of opportunities for going off road and jump around on bumps and roots. This part can be quite busy, so make sure you watch out for other users.
After crossing Sieveringer Strasse, the path continues to head back to the starting point. The part along the little stream in its concrete bed is quite nice, and a good opportunity for giving muddy tires a good clean (don't hesitate to ride into the stream for a bit).
Later on, the trail meets civilisation again and it's time to start thinking about how to get home. The end point is at the beginning of Czartoryskigasse again, just because I live in this area. There is not particular interest in ending the trail there.