I suggest you run a few different speed tests. Wally B posted a link to one site. Run that, then run from some other sites so you have a consensus speed and can call your provider and compare the speeds you're seeing with what you're paying for. Distance to the CO does matter, but if you're over the limit they shouldn't have sold it to you. It could be a simple problem on the copper back to the CO (splice tap, impedance, etc.) that is slowing you down. They can test to your house and see if the problem is in your home wiring or on their side of the circuit.
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Mike