The Victoria Rail Trail is an abandoned railway which has been transformed into a 90km recreational trail.
The main section runs from Lindsay to Haliburton along the former Victoria Railway right-of-way. There is another southern section that runs from Lindsay down to Bethany.
Along the way the trail crosses old railway trestles, goes by abandoned stations, and there's even a model railway museum in Kinmount. It's neat stuff if you are a train geek!
As a cycling challenge, it's not especially difficult - the trails are mostly hard-packed gravel, and the hills are all gradual. The distance is long, and much of it is far removed from civilization (plan for emergencies wisely!). It's not suitable for road bikes, but anything else could get the job done.
Ken Brown has put up some details and pictures on his web site.
I have done everything from Bethany up to Kinmount. I suspect the most interesting sections are further north, so next year... to Haliburton!