Find English speakers on italki who want to learn Mandarin Chinese (there are tons of them). Contact them and set up language exchanges on Skype. You'll learn how to speak most quickly by, well, speaking. And you can ask them to correct any particularly egregious mispronunciations, too. It will take some time to get one of the "natural English" accents, so I'd focus on fluency before voice mimicry, but simply practicing with natives will help you to do both.