According to the forum, it should take about 11 days from Porto. The John Brierley guide, Camino Portugues, which I will be using, divides the walk from Lisbon into 23 days. You might find the walking from Lisbon discussion interesting.
Jonathan, I walked the Camino in 2012 at age 61 in 43 days. I had shin splints and took three days off before continuing. At first I took it easy, by the end, I was hiking 42 km/day. I recommend starting slowing and increasing the distance as your body adjust. I had two additional rest days.
Don't let anyone disuade you from the Camino—it is a experience of a life time. If you don't have time to complete the entire trip. Start somewhere closer to Santiago.