Silke,
There are some great trails of the north side of Mugdock, which isn't actually in the park itself, and further to the northwest which all link into the west highland way. This is great singletrack stuff but you will need to have decent mud tyres for that stuff. You can either park your car at Mugdock itself or Milngavie for those routes.
I took Gordy and Adam on a couple of runs there so ask them what it's like.
Some of the routes also give you a great view down to Loch / Ben Lomond and further afield....... not just over spam valley!
You can also do loads of loop variations from Mugdock alone including taking in Lennox Forest to the east.
Have fun!
Fraser