If you want to aim for a marathon this summer, I would start building your base mileage with the marathon training program. This may come at the sacrifice of also aiming to lower your 10K time, as you will be focusing on building that base mileage, rather than speed.
Alternatively, you could do the 10K program, and then switch to the Half marathon program. That would give you time to build the mileage after you have done your 10K.
You might check over on the e-chi training forum to see what advice they offer. Hope that helps!
Certified Chi Running and Chi Walking Instructor