Hill1 - I usually do Edinburgh plus one other spring marathon, this year, like you, I did London. Edinburgh is always late so the gap is big enough that your legs will be fully recovered but small enough that you can basically rely on your fitness from your earlier marathon (there's not really time to get more than one proper long run in between your recovery and taper). I don't think I have ever managed to improve on my previous spring performance in Edinburgh and I won't be trying this year, I'll probably run around 10 secs/mile slower than I did in London and try to enjoy it.
P&D says "These sessions are done after a thorough warm-up and often towards the end of a general aerobic run or recovery run. Allow yourself plenty of time between repetitions so that you can run each each one with good technique... A typical rest is to jog and walk 100 to 200 metres between repetitions."
Personally I run them so they're all done in time for a 1 mile-ish cool down, 100m on, 100m(ish) off (just linearly as part of my usual out and back runs, no backtracking or messing around on a track).