The anxiety, doubts, and trepidation before embarking on the Camino are absolutely normal and even necessary. DO NOT let these feelings hold you back and use them to get you focus and determination. I never walked more than 5 miles at a time in my life and yet completed the first day SJPDP to Roncesvalles in less than 8 hours and felt amazing. The climb on this route is somewhat gradual, so if you walk slow as you go up, you don't really feel the climb. DO NOT RACE, use a baggage transfer service on this first day, and take breaks at least every 2 hours.
Definitely whether is March or August the stop at the SJPDP Pilgrims Office is a MUST. They have very knowledgeable, local volunteers that will answer any questions, including whether the Pass is even open, it may not be and you may have no choice, but to go the Valcarlos Route. Honestly, in March, I would plan to go the Valcarlos route, regardless.