In reply to IainRUK:
I used to drive from Paderborn to Oberstdorf on the German/Austrian border most weekends. Plenty of mountains there for you to run around on. There is also the Karwendelbirge just further on.
Other alternative, is to head to Garmisch Partenkirchen around the Zugspitze area, lovely place and plenty of things to do.
Locally, if you are in Northern Germany and you don't want to travel too far, you can go to the Harz mountains. Lower level but I'm sure you can beast yourself on the trails!
Have a good time Ian.