What about the highways that are being built outside of the Warsaw area
Unfortunately non of these roads (S7, S8) will be finished as soon as 2014 or later...
S8 is a motorway only to Wyszków (about 60 km from Warsaw) then it is a normal road packed with TIRs from Baltic States...
I am using it to get into Eastern Mazury (Ełk, Augustów) but you have to turn north in Ostrów Mazowiecka towards £omża, and then you follow 61...
S7 is a motorway only to Płońsk (60km from Warsaw), then it is also a normal road crowded with tourists heading from and towards seaside.
I would recommend you two routes which I've been using a few times this summer:
61 to Pułtusk, then 57 to Szczytno, then 600 or 58 and 59 to Mrągowo or
61 to Ostrołeka then 53 to Rozogi and 59 to Mrągowo