This is all great feedback guys/gals - thank you. Perhaps I should fill in some of the blanks to asssit those offering guidance.
My goal is to run 10miles comfortably (my main reason for this is so that I can run 5miles to a partners house, spend the evening there and then run 5miles back later that evening - this will allow me to keep my running up through the winter as the route is lit and is convenient). I recon that if I can run 10 miles comfortably then 10 miles with a gap in between should be within my reach.
I currently run 3 times a week (Mon, Wed & Fri) and then try to fit in a longer run at the weekend either Sat/Sun depending on how I feel and the way my weekend pans out. I recognise that this nearly always means that I am running two days on the trot either Fri/Sat or Sun/Mon (not ideal, I know) but it seems to fit in with my life style that way and maximises the time I get out running.
I am looking to do the interval work simply to vary my running, to add a little variety and to keep me focused and motivated as well as hopefully improving my speed and stamina. I like to track my performance.
To give a little feedback on performance so far: (All data according to Nike+)
01-31 July 13 Runs, Av Dist of 3.0miles, Av Pace: 11'08"
01-31 Aug 15 Runs, Av Dist of 3.1miels, Av Pace: 10'33"
01-30 Sep 9 Runs, Av Dsit of 4.2miles, Av Pace: 10'52" (to date figures)
Not sure what that tells me exactly, but there they are. I have much greater detail on each run of course but didn't want to go mad. 
Thanks everyone for the info, really appreciated.
C.
Edited: 17/09/2012 at 13:32