I was thinking that I could firstly use the GPS to get the user's start position at 0 seconds (t = 0). Calculate the user's new psoition at a time interval (i.e. every 5 seconds) adding the distance travelled in that time to the original distance. I would repeat this loop until the user was finished their session.
However I am not the greatest programmer in the world. Therfore, any advice you could give me on coding this, presumably, in Javacript would be fantastic.