The Camden Cycleway is only about 3.5-4km long, depending on if you go down to the little jetty (which I'm not sure you're allowed to do on a bike. I seem to remember a sign). It starts a bit further along than the end of Ulmarra Ave (which is where the MTB track starts) on Martine Rd and follows the river along to Camden Valley Way. A road bike or MTB are fine for this cycleway.
It's a shared concrete cycleway/walkway that gets pretty busy on the weekends. It is especially busy near the soccer fields when games are being played. People are out cycling, running and walking dogs all day. It's a fairly flat track with only the occasional dip to ride down and up and no challenging inclines at all.
It's pretty easy to get a bit of speed up but there has always been some loose stuff on the bends to make them a bit slippery when I've ridden along the track. And lot's of people that you don't want to crash into. Finishing at the Camden Valley Way end you have three good options.
The first option is to turn right along Camden Valley Way and within a minute or two you should see the BMX track on the right. You could muck around for a bit in there and then head back.
Option two is to turn around and ride back the way you came.
Option three is the best and only adds about 10 minutes of ride time. Turn left and head into the the main street of Camden. After the second round-about and just after the Kum Hor chinese restaurant is a great little deli type place that has an even better lollies and ice-cream shop upstairs! Grab an ice-cream or thickshake or both, then sprint back to burn off what you just ate!