For training rides (whether or not you actually race), it would be irresponsible to ride on an MUT at 30+kph what with all the children/strollers about. But my wife is scares to death of riding on the streets (I've been hit by cars in T.O three times) and the MUT's are a godsend.

Here's an awesome North-of-the-city ride. Go up Warden to Old Homestead Rd, turn right and go to Pefferlaw and back. It's a 100+ km ride along good roads devoid of traffic and avoids all of the really leg-breaking hills in that area.