Travel to enjoy Alentejo by bike. I recommend, at least, 4 days. You can do it in more days stopping and visiting the places along it (beaches, castles, monuments, restaurants, scenic views, ...).
Small traffic, good pavement, highway when the national road is not interesting (Alcácer do Sal to Lisbon, for example, where the national road has lots of 50Km/h traffic lights and lots of slow traffic).
In this roads (most of them at least) we can go faster or slower at your own speed of comfort.
It is possible to do it all or only parts of it, for example, you may not go to Cape Espichel and Troia.
I recommend that you do and stay at Lisbon –Troia – Minas S. Domingos - Elvas - Lisbon. You can stay more than one day at each place to explore and rest.
Note that if I would go by car maybe I would use shorter routes.