Fatima is doable in one day.
It is not impossible that the weather in Feb will be reasonable but it will not be warm. Remember that Lisbon/Cascais faces the open Atlantic. There are a couple of small beaches at Cascais but for easy access you should consider Carcavelos and Estoril Beaches.
For an idea of average temperatures in February, see:
There are, at least, two direct routes by public transport to Fatima, both by bus:
a) Rede Expressos - http://www.rede-expressos.pt/
From Lisbon Sete Rios Bus Station, (Metro Jardim Zoologico) a Rede Expressos bus to Fatima. There are regular departures through out the day between 07:00 and 20:00. Transit time is 90min.
b) Citi-Express - http://www.citiexpress.eu
Citi-Express also operate from Lisbon Sete Rios Bus Station, (Metro Jardim Zoologico) and Lisbon Gare Oriente (Metro Gare Oriente) to Fatima 2 to 4 times daily. Services on this company's lines 4,5 and 7 depart from the Bus Station at Gare Oriente. Tickets are available from bus ticket office 8.
Timetables here: http://goo.gl/8ZuB4
It is not recommended taking trains to Fatima as the station at Caxarias is several kilometres from the sanctuary and local buses are not frequent (max. 7 per day).
There are also day tours to Fatima. See, for example:
