As someone who ran a 4.6 in my senior high schol year, let me tell you this:
Unfortunately, the bulk of "speed" as we know it is mostly a gift you're either given or not given. You can control this and expand on it with training and stuff like that, but the chances of you improving dramatically through training are minimal, unless you are an out-of-shape slob.
Personally, I never put stock into these types of devices. Some people claim they work but my personal feeling is that they are just a marketing ploy which looks good when advertized (stuff like "Bow-Flex", for example). The best training to increase cardiovascular endurance and speed is good old plain running. Try something like going on the treadmill and running for 60 seconds, then slowing to a jog for 60 seconds and do it 20 times. As your body gets more comfortable, play around with it, increase the length, etc... to put more demand on your body.
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