my "favourite" sessions (i.e. the ones that make me hurt in the morning) are:
9 x (2 mins @ HMP + 1 min rec + 2 mins @ 10kP + 1 min rec)
20 x (400m or 1/4 mile @ 10k pace or faster + 1 min rec)
6 x 3 mins @ 10kP + 2 min rec (undulating course)
5 x 1 mile @ 10kP +1/4 mile rec
Week 1: 20 mins @ MP + 20 mins @ HMP + 20 mins @ 10kP (continuous)
Week 2: 30 mins @ MP + 30 mins @ HMP + 30 mins @ 10kP (continuous)
Week 3: 40 mins @ MP + 40 mins @ HMP + 40 mins @ 10kP (continuous)
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The basic purpose of intervals is to get used to running at faster than normal paces to make running (movement and breathing) more efficient and to get used to running at a faster pace. I would prefer to do the fast runs for a couple of weeks, then move onto the longer ones to make my body used to running at 10k pace in a run that overall, is longer than 10k. Increasing 1 mile reps to 1.25, 1.5, 2 mile reps means getting used to it so that in a 10k race you can either do that pace easily or better, go faster!
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MP = marathon pace (PB or target time)
HMP = half marathon pace (PB or target time)
10kP = 10k pace (PB or target time)