For me, 1k reps are key to a swift improvement. Try and run at your target pace, or close to it. So, aim for 4.30, rest for at least two mins and then try and do another one. Ultimately, you aim to do about six with a short rest between each.
That said, upping your mileage is very important too.
Your body ought to tell you if the shoes are done. Personally I get good use up to around 350 miles from my Brooks but then I`m fairly big (6`3 and 12 stone). After that sort of distance I know I am risking it doing LDRs or real quality stuff so I use them for shorter runs or off-road until 500 miles or thereabouts.
Also if I was you, I`d put the Cortanas on ebay as plenty of folk would probably like to give them a whirl.