It can be modified to make it longer or shorter.  In clear weather the views to Ben Alder and Loch Rannoch are worth every pedal stroke.
Carie lies around 3.5 miles west of Kinloch Rannoch on the south side of Loch Rannoch.  The area is rich in natural and cultural history on top of the rather gorgeous scenery and offers a lifetime of exploration!
This route largely follows the posted yellow route but leaves it for a while before joining it on the return.  Noting the yellow-marked posts, cycle up out of the car park following the track upstream.  Don't cross the bridge, instead continue upstream for a few metres and the track heads away from the burn towards a large shelter but veers right before reaching it to join a land rover track.  Turn left and head up this track. This is the yellow trail.  You follow the, very straight, gorge of the Carie Burn uphill until you come to a junction.  Turn right and follow the yellow posts on the fire road.  Shortly you'll see a lochan to your left (it's partially obscured by trees but you can get views of it from a footpath right at the end of the wall).  I think that this lochan was formed as part of a scheme in the early 1800s to float timber down to the loch and thence by the Rivers Tummel and Tay to Perth.
Continue cycling down the fire road and you'll see that the yellow trail heads off into the woods.  This route remains on the fire road for the next few kilometres veering round to the south west and crossing the top of the Dall Burn.  At the next junction turn right.  This eventually takes you to a previous part of the route only this time you are heading back and downhill towards the main road (avoid turning left down the track to Dall, if you do then just cycle back to Carie along thje lochside road).  After a few kilometres, you will see the yellow post marking the singletrack path back to Carie.  This is lovely to cycle on, just watch out for the rather unfriendly water-bar drains.  There's a lovely spot to sit and look at the view from amongst the oaks (darach is gaelic for oak).  Soon you'll re-cross the land rover track and follow the path down to the car park.  Refreshments and a really useful shop are available in Kinloch Rannoch. Tips:
The best map for this route is OS Explorer 385 Rannoch Moor and Ben Alder
The FC Kilvrecht Camp Site is minutes away.
The nearest bike shop is Escape Route about 24 miles away in Pitlochry, http://www.escape-route.co.uk/.
The roads and trails in this area tend to be quiet.  So, they are eminently suitable for all types of cycling.  For example, the circuit of Loch Rannoch is popular and very rewarding. 
The whole of Highland Perthshire is great for mountain and road cycling.  Info is available from Escape Route, Highland Safaris near Aberfeldy http://www.highlandsafaris.net/ and Progression Bikes in Dunkeld http://www.progressionbikesscotland.com/ .