First of all you need to work out your pacing for the various types of run (fast, slow, easy, marathon, tempo etc etc) - there's a pace calculator on this site to help you.
The 7 mile warm up is probably going to be at a fairly east pace, before you move into the fast/slow intervals.
Don't worry too much about making the distances accurate, just work out how long roughly a mile should take at each pace (which the calculator will tell you). I use a watch that I can set to beep at different intervals, which helps with this sort of thing.
Hope this helps. I'm not an expert on these things by any means, and I'm sure other forumites can chip in with info and ideas.