I really depends on how much time you missed because of the sickness.
Obviously you missed your maximum potential peak since you planned the taper for a particular date. However, you still have the fitness gains from that training cycle. Plus you don't have to recover from a long and hard effort like a marathon. The hardest part is convincing yourself that you can feel fresh so soon and realistically aim for a PB.
Now, I'm familiar with Pitzer's plan. In your situation I would do something similar than the last 6 weeks with the following changes:
-First 2 weeks take it easier if you need to.
-Cut a little on the biggest mileage week with the 22 miles long run.
-A little less taper, since you have less to recover from.
-Keep the intensity in the workouts.
-Plan a slightly slower pace for the first half of your marathon. Chances are you'll go out too fast anyway, we all do.
If you missed 2 months or you don't feel right jumping back in a typical Pitzer's week then forget about being top shape in April.