Well marked though remarkably difficult on the eyes, the camino turns inland at Sacavém (itself nothing more than an extension of Lisbon). The park ends in a disused parking lot (except for when the Circus is in town) and follows a wide joggers path along the canal.
You will follow this canal inland, around a heavily developed hillside along the river Trancao, and then back to the Tajo river. There was construction along the way in early 2016 shortly after leaving the park behind you.
To keep on the camino:
Keep straight through the first roundabout and pass under the National highway; there are several roads here and directly underneath the second one is a popular fishing spot.
At the next intersection, turn Right to cross the river (this bridge is where the construction is happening).
Once on the other side, keep a look out for arrows directing you to cross the road.
From there, seek out the path that follows along the river bank.
Technically you have done nothing more but cross sides of the river, but it would be easy to keep on the road. Yellow X's warn against that.
In a short distance, you will pass underneath the very high bridge that carries the A1 highway.
Between here and Granja, you will keep the highway and hillside on your right and the river on your left. Apart from a Quinta in shambles, there is little on the trail.