Arrive in Lugo and start walking from Sarria the same day

If you get off the stop before Lugo you will find it is Sarria?

Otherwise you could always walk from Lugo to Santiago away from the crowds unless you really want to do the Francès?

Rioja Routard is correct:

STATION ARRIVAL DEPARTURE
MADRID-CHAMARTIN 22.30
AVILA 00.00 00.02
MEDINA DEL CAMPO 00.42 00.51
VALLADOLID-CAMPO GRANDE 01.20 01.22
VENTA DE BANOS 01.46 01.47
PALENCIA 01.55 01.57
LEON 02.58 03.00
VEGUELLINA 03.19 03.20
ASTORGA 03.30 03.31
BEMBIBRE 04.12 04.13
PONFERRADA 04.28 04.29
O BARCO DE VALDEORRAS 05.03 05.04
A RUA-PETIN 05.13 05.14
SAN CLODIO-QUIROGA 05.33 05.34
MONFORTE DE LEMOS 05.56 06.11
SARRIA 06.37 06.38
LUGO 07.04 07.06
GUITIRIZ 07.34 07.35
CURTIS 07.57 07.58
BETANZOS-INFESTA 08.24 08.25
BETANZOS-CIDADE 08.32 08.33
PONTEDEUME 08.50 08.51
FERROL 09.13

Travel time between MADRID-CHAMARTIN and LUGO: 8 hrs. and 34 min.

Going to Lugo on the train would involve not-getting-off in Sarria.