Cathyke should pace herself, feeling free to stop for cafe con leche, and enjoy the spectacular countryside. 21km should take 5-7 hours, as one should be sensible on the first day. Be hydrated. Wherever you stay in Rabanal (I've stayed at both the refugio de Guacelmo run by the Confraternity, which featured in 2002 the most wonderful hospitaleros I have seen, encouraging and soothing and caring for pilgrims; and in 2005 at the hotel El Refugio), make a point of attending vespers with the Benedictines. It will be a memorable first day and will stay with you forever.
I should note that, in 2005, I was about to step out from the bar in the Hotel Gaudi across from the Cathedral into the dark when the Vicar-General, dropping in for a coffee after the 6.00 am Mass, insisted that I take a stirrup cup of brandy in my cafe con leche. I am not accustomed to drinking at breakfast, but found that it fueled me wonderfully, propelling me up the Camino for the next few hours. I accepted only because one should not discourage the clergy from buying drinks for pilgrims.