I'd go ahead and make the reservation. Inquire about cancelling if weather makes it impossible to go that route. As you take those first steps to Orrison..it will be hard, steep and trying. ..but so worth it. I kept telling myself that those first few days areally really known to be the hardest. No matter how much training before you leave ...it's hard. Embrace it...each day will make you stronger...allow the experience to pull you forward. Use walking sticks, they truly help. Talk to pilgrims, share stories, converse with locals, just take it all in. Read evaluate your backpack contents as you go. If your pack feels heavy eliminate items. Mine started at 21 lbs on the third day 10 by day 15.... 6 lbs. Nothing will be so precious to keep if it prevents finishing. Ok I never let go of my favorite hairbrush or my real full size beach towel. And take care of your feet..Rest them ...pamper them with freedom from your socks and shoes several times daily....they deserve it...you can't make it without them. It was the hardest most enriching experience of my life...grab it and enjoy.
Buen Camino