I'm going to be a dissenting voice on the Spin classes. In my experience, unless you find one that is really designed for cyclists, the program is not specific enough for race training.
Also, they tend to gravitate toward shorter more intense efforts, wheras what you're going to want for time trial preperation is longer intervals, such as 2x20 at threshold.
Spin class once a week isn't going to hurt you, and its better than the couch. But, assuming you're self motivated, a specifically designed program using a trainer is going to give you more benefit than spin class.
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You could get lost and die.
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