That is a very good plan, but I would pass Santarém and instead, take the train until Tomar. It's preferable, and it has many more things for you to see.
I think that for Lisbon, if you just stay 2 days (counting with the night of your arrival) it will not be enough
Coimbra also worth’s the trip, trust me. Don't forget to visit the Biblioteca Joanina (Joanina Library), to see one of the best book collections in the world (they even have books covered in gold!).
About the train… The 50% discount it's only valid for people over 65 years old. So sorry mate, no can do. They have now a special campaign that if you buy your ticket untills 5 days before your trip, they will make a 40% discount! You can check prices, schedules and other information’s in here: cp.pt
If you need something in Lisbon, just ask ok?
Best Regards
Diogo