Hi Jimmy,
you should be able to complete the walk from Orisson in about five hours or so, that's at a comfortable pace and stopping for rests along the way. So getting a bed in Roncevalles shouldn't be a problem. Do take care on the descent into Roncevalles though as it can be very hard on the knees.
You should buy some food for the walk in St Jean the day before but failing that there was gentleman selling drinks, energy bars and chocolate out of a van when I walked the last time.
From experience, there is a shortage of water fonts between Larrasoaña and Arré which would most likely be on your fourth day. There's one in the car park of a public park, just off the Camino trail that I missed when I walked last August. Unfortunately that meant that I ended up short on drinking water and walking in the full heat of the afternoon sun with very little water. It's not something that I'd recommend. Again, there was a very enterprising man selling drinks further on so I'm sure that I'm not the first to be caught out like that.
If you take your time at the beginning, you'll find yourself growing in strength and better able to walk further should you want or need to later on. You'll also get a better sense of your physical limits and know when to push yourself and when to rest.
Good luck and Buen Camino!
Darran