It would be great to have complementary Increment and Decrement methods that refresh the expiration. Unfortunately, it's not safe to do a Replace(Get(), ...) as that would be a race (another goroutine could change the value between the time the Get() and the Replace() happens).

Perhaps just a new method to UpdateExpiration on a key would be sufficient, here? You could pass a time, or DefaultExpiration, or NoExpiration as appropriate. So:

func (c Cache) UpdateExpiration(k string, d time.Duration)

Related: PR #20.