I walked to Compostela via Lourdes in 2005, stopping off in Lourdes for 2.5 days.
If my knee could let me walk the Camino again, that's where I'd start.
I would definitely suggest NOT following the "official" route from there, especially in December, but to stick to the small départementales roads in the foothills -- if you do this, it's not that difficult (though you'll need your woollies).
I'd suggest the couple following things --
1) Lourdes is a pilgrim's destination in itself, so if you start from there, plan on staying at least two nights both in celebration of completing this first pilgrimage, and in preparation for the further pilgrimage to Compostela.
2) Get your first stamp on your Credencial at the PARISH Church, not the Grotto -- but get your stamp from there AS WELL (it's a particularly elegant one
), as well as from whichever hostelry you end up in -- unlike the Lourdes or Rome or other pilgrim destinations, the Santiago route is technically from parish to parish, until the cathedral parish of Santiago -- whereas all other Christian pilgrimages are far more clearly focused on the destinations, so that however you get there (walking, cycling, bus, car, aeroplane) is far less important than on the Camino.
erm, I'm not sure if I can remember enough to recommend any eateries frequented by the Lourdais themselves, rather than the religious tourists, but good luck on that score !!!
Otherwise, you should be able to avoid any severely wintry conditions except for the actual crossing into Spain, whether you choose the Somport route or the SJPP one.
Ultreia !!! And may our Lady of Lourdes bless your pilgrimage.