An alternate strategy:
There were and probably still are frequent and economical commuter trains alongside The Way between Lisboa and (I think it was) Azambuja, with many intermediate stations right on the Camino. One can walk "Camino north" (which here is north-east), with a day pack as far one wishes on day 1, then hop on a commuter train back to the big city to return to your pre-booked lodgings, rest up overnight, then on day 2 visit city museums etc., on day 3 take a commuter train to your previous stopping point and walk north again for a day, spend day 4 back in Lisboa, repeat cycle until you run out of commuter trains.
This is in no sense "cheating": you will still be walking every inch of The Way.
Doing this for the first few days provides a gentle way to build up walking fitness and avoid the problems of finding lodgings along this first section, whilst getting to know Lisboa. It also provides the stability of a fixed base in Lisboa for a few nights. Lisboa provides a vastly wider selection of good restaurants and other infrastructure such as laundromats than does The Way.
Lisboa and Sintra (a short train ride away to the west) are stuffed full of really really interesting places to explore or just for walking around the neighbourhoods.
I spent a full a week in Lisboa and Sintra before my northwards walk and a second week there after completing my pilgrimage. Each of those 14 days in Lisboa and Sintra was time well spent, and I still want to go back for more.
Lisbon commuter train schedules:
Lisbon Urban Trains let you travel quickly and cheaply and connect up with other CP services or road services. Further details.
www.cp.pt
Frequent Metro trains run to the north of Porto for a few days' walk alongside The Way too.
Metro do Porto - A Vida em Movimento
en.metrodoporto.pt