I did the 60 miles route last year and it was a good ride. If it is the same route as last time then it goes up the Dukes Pass the other way to the way on the Scottish bike show route. It is still quite a climb but slightly easier because it is less steep and the descent is really enjoyable because of the scenary and the nice tarmac - although when you get to the hairpins you have to be quite careful particularly if wet. It is a figure of 8 route so you do the Dukes twice on the 60 and 90 mile route...both routes are the same up until about 50 miles then they split after Aberfoyle.
The road around loch katrine is really nice, it is only open to access for cars of residents so is very quiet (most times ive been along ive not seen any cars).