I do my hill sprints as part of a 3 mile run, just about in the middle.  I agree that a good warm up and cool down are essential for intervals.  I'm not sure what your experience level is so I'll throw in some info that might help, but with a 2:08 half marathon time you're doing well already.  Maybe someone else can benefit as well.

Speed intervals are intended to lift your maximum aerobic performance level.  The idea is to push yourself just into the anarobic region where you're muscles are starting to burn and you're producing more lactic acid than your muscles can get rid of.  By doing this you'll get better at removing the acid and be able to run faster for longer.  Shorter intervals, typically a minute or less, will target your threshold while longer ones at race pace will tend to target your overall endurance speed.  The chart you show will build a higher threshold early on and then use it to improve your overall race pace which is probably just what you're after. 

One thing worth mentioning about sprints and intervals.  Don't do them in the same workout as longer endurance training because they're aiming for different results.  Long runs at endurance pace will improve your mitochondria production in your muscles which is essential for the energy needed on long workouts.  Training at levels that produce lactic acid destroys that new mitochondria growth and is therefore counter productive.  I deliberately do speed work on week days and endurance work on weekends. 

A friend is a professional cycling coach.  He recommends a 2x20 minute interval at 75% effort with a 3 minute recovery for endurance building and for speed work it's 8x30 second high intensity (90 to 100%) effort with 1:30 to recover.  By the last interval you won't be recovering much at all and it's not uncommon for amateurs to only do 5 or 6 intervals so a lower intensity works too.  Any recovery past 3 minutes is too long because your heart rate should be down as much as it's going to be at that point.

Best of luck with your racing this year.

Andy

Definitely keep them separate to your longer runs as the idea is to hit these sessions feeling fresh so that you can kick up and maintain the intensity of the session.

Looks like a good plan though. I was down the track yesterday and managed 8km of sprinting in total....it was tough!

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