Leading out from Orillia there are a few old rail trails that are fun to ride, especially if you have a cyclocross bike and can put the hammer down to ride at speed for a while.
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This morning I went north out of Orillia and rode the Uhthoff Trail to a town called Coldwater, which is a 25km trip one way.
The ride is dead flat the entire way. It's also pretty much dead straight - so it isn't exactly the most interesting ride in the world. However, it is really nice to be cycling all alone through forests and past farmland for two hours.
In places the trail is a bit overgrown. It's a shame, but I don't think this trail gets used very much, not like the Oro-Medonte trail which goes between Barrie and Orillia. After turning around in Coldwater, and starting the ride back to Orillia, I passed two ladies on bikes who were going to give up with their ride - worried that there was actually a lot of poison ivy in the overgrown sections.
If there is poison ivy in there (and it's very possible), I might be immune because I got through without lots of scratching occurring later in the day. Actually, I also scared up a skunk this morning, which I was lucky to encounter when I was riding fast, and he didn't have time to spray me.
So, this trail isn't perfect, but it is worth doing. It's listed on the Huronia Trails and Greenways website, and I also found another gentleman talking about the trail on his website.