How to structure base-building before Pfitzinger's 18-week plan?

I think you definitely want to get yourself ready for what the plan throws at you by modelling your base building on Pfitz-style training.

If you go buy Faster Road Running he does this for you with several base-building schedules, but you can also just figure it out on your own. You want to be doing a few medium-length (like 20% of weekly mileage) moderate/easy pace runs, a shorter recovery day or two, and long run that progresses from easy to about 30-sec slower than marathon pace. Every other week throw in a tempo run for anywhere from 18-22 minutes at about 10-sec slower than 10K pace. Bump up weekly mileage every week or so with a pause/cutback week every 3-4 weeks.

If you look at the 18/70 schedule, I think the first day is a 4 mile tempo. If all you do is easy running during your base phase that's going to be a rough day. Also, Pfitzinger doesn't really do any slow long runs--they're all some form of progression that has you finishing relatively fast. So if you're not doing those they can beat you up.

Thanks man (no sarcasm).

LRC can be full of trolls and stupid posts, but the training threads are quite useful!

If you are building mileage with an intense plan on the horizon, then I wouldn't worry too much about anything other than getting comfortable with the increase in miles. It follows the adage of only increasing one stressor at a time (i.e. mileage).

But you don't have to be bound to only slow, easy running during base. Just run a little harder on days when you feel good or do some start easy, finish faster days, but don't plan ahead. Decide when you're out there and feeling good.