If by Park you mean Park Way....
Walker - heading from Hillsboro east... I skip Walker altogether - Evergreen, turn east on Cornell, then south on 158th IMMEDIATELY get into the left turn lane, turn into the office park by the gas station.
The next part is hard to explain in email - follow that street to the end, turn left. Then turn right, then left, which takes you into the north entrance to the Tualatin Hills Park and Rec complex. The soccer fields are on the left side. Go 3? soccer fields in and turn left (you are on a paved path all the way. If you have to cross a creek, you've gone too far). Follow the path and you'll pop out onto Pioneer.
Continue on Pioneer until you get to Meadow, turn right. Turn left on Downing. Note: you can turn LEFT onto Meadow from Walker; it is the street right across from the Nike entrance, so there is a protected left and a light.
At the end of Downing, bear left onto Butner. Cross Murray at the light. Follow Butner until Huntington, turn right. The road goes down, then up (wheee!). Turn left onto Park Way. You can take it from there.
Alternatively, if you must ride on Walker, turn left at Meadow. Or left onto Filmont; take the third right (Faircrest) follow it all the way and it drops you onto Park Way. Filmont is NOT a protected left. I try to hit it when the traffic is heavy, so I can merge in and turn. Agressive left turn signalling has also been known to work (GloGlov aided). If that fails, I stand by the side of the road and stick my hand out. It eventually guilts someone into letting me cross.