I don't know the bit from Lourdes to Argeles-Gazost, but from Argeles onward it goes up the Gorge de Luz and it's only a very slight upward gradient. Take it easy and you'll be just fine.
Of course, when you get to Luz St Sauveur the easy ride stops and it's turn left & game on
PS: There's a cycle path callled the greenway (I think) that runs from Lourdes to Argeles. I haven't ridden it, but I think it's pretty much flat.