Here is a good Distance route.
Start at the open castle green.
Go all the way down the mountain.
Turn left on Lower Cliff Trail.
Turn right on Iron Bridge Trail.
Turn left on the Black Road.
After going round the first bend in the road at the big downhill to town, keep left.
When the road curves for the second time, keep going straight.
Go over the red ramp that you see.
Make a slight left.
Go to the Start circle.
Turn left on River Trail and when it ends you have completed the loop.
If you want a higher chance of starting your next cycle at the Start line, when you have a few minutes left on the clock, just keep riding the inital Short running circuit. This will slightly reduce distance though.
Total Distance:20-23 km and 10-12 km at 15 minute message. This avoids slowing down on Red Iron Bridge.