Is better to leave line 3 (green) at Vallcarca instead of Lesseps. It's closer and from there, you can take the escalators that connect metro station with park's west entrance.
If you want to walk even less, leave the Green line at Passeig de Grącia and take the bus #24, that has a stop next to the park's east entrance
(but be careful with pickpockets)
As GK says, a T10 is the better way to save money. It allows you to change for free from metro to bus (and vice versa) or from bus to bus within 75 minutes from first ride. A single ticket doesn't allow connections.
Park is open longer that guides say (my gym is next to east entrance, and you can go jogging to the park from 8am)
Sitges is better reached by train. Until the general election day (9th March), commuter trains to the airport and south of BCN (Sitges and Vilanova) are free, because services are affected by the works of a new high speed line. There is not a clear schedule for the return of paid journeys.