Your current approach sounds good. 10-15 miles a week is good. I try to dedicate one day of the week to speed, and that usually is the day I run intervals. As you run, try to take like 30 second intervals and double time, maybe once every 2 minutes double time for 30 seconds. What I mean by double time is not double your speed, but some definite increase that makes your body work for 30 seconds.
A big thing to remember, shaving time off any run takes time and effort and lots of both. 10 seconds a month is an awesome goal. Ive seen much more, then again ive seen people(myself) get slower as they up their mileage. Settle into something that works for you. Sprints help a lot but they are a great way to get injured. Moderation is the key with sprinting.
A little off tangent here - but cross training also helps. Spinning/Cycling or a light weight lifting routine helps. Anything to help build the fast twitch fibers in your legs will totally help with your speed and muscle endurance. (enabling your muscles to hold that pace for extended periods)
Also, i dont know if you have access to a treadmill, but treadmills are awesome for working on running cadence. Lets you get the feel of what a 6:30 - 7 min mile cadence feels like.
Check out http://www.runnersworld.com
There is tons of info there about run training, regiments and everything else about running.
You've got the hardest part beat and thats your mind. You obviously want to improve and are making the effort. Stick to it, it will come.
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