Minni, hope you feel better soon.
I sometimes look at Macmillan pacing table, usually if I'm wondering what my rep times come out if I want to achieve a new target such as 40 min 10K for example, then I can see how much quicker I would need to be in my intervals. I agree with Morgahan that you don't necessarily slow down on longer reps as the purpose of the reps can change.
However on this thread we are talking about marathon training, out and out speed is not a issue but being able to race shorter distances at considerably faster than MP is a issue. Relistically Minni is not going to race a distance shorter than 10K between now and her marathon therefore as Moraghan has suggested above there is probably not much point in getting too hung up on VO2 workouts.
P & D has VO2 and LT sessions on alternate weeks, I think you should be doing one of each every week but lets stick with the schedule! I think for the sake of simplicity if we assume that LT runs should be at 1/2M pace which for Minni should be about 7-30 pace, and the VO2 or interval sessions at around 5K pace or 7-00 pace. The schedule uses 600 /800/1200/1600 intervals I don't think it actually matters a damn what distance you use and that the variance is more for the sake of variety than anything else.
I always use 800 reps as my reference point even though I do lots of various speed sessions, I start the season running 6 x 800 @ 5K pace off 2 minute recoveries, build to 10 x 800 at same pace and recoveries, then start reducing the recoveries until I get to 1 minute and thereafter increase the pace. Because I have been using this method of gauging my fitness for years I have a pretty good idea of what I'm capable of race wise at anytime.
Some books will tell you that about 6 weeks of interval work will prep you for a race, I prefer to run speedwork every week for around 9 months of the year but as i say its a gradual build up over the period but then I race a lot over distances from 800M to ultras.
So in short Minni you can stick to the schedule or use any distance of rep you want for speedwork, whats important is to see progression and as your speedwork period is quite small it might be better to stick to one distance to be able to gauge progress more easily. Sorry to go on about this but you really need to get those tempo and interval sessions done at around the paces I'm talking about here. What we want is that you feel very comfortable at MP.