Maybe the wrong section to post but here goes. I have a iphone 4S I also do a lot of running, I'm thinking of purchasing the Nike + sensor, being that I have the 4s would I just need to purchase a sensor or would I need to purchase the whole kit? Which I believe has the module to plug into the nano ?

You just need the shoe sensor. I also have a 4S and have used it may self. You may be interested to know about products like
http://www.amazon.co.uk/Grantwood-Th.../dp/B000PGQ1AC
Which allows you to use 'normal' trainers/shoes and not be restricted to the Nike+ trainers.

you might find that an app called RunKeeper is much better than the Nike+ app, and the app is free.