There are many discussions about the Portuguese Camino routes, which way to go, coastal or traditional inland. The level of difficulty, options and variants and the concern for walking on sand on the coastal way. My typical walk is 15-20 km per day and I try and stay in hostels and small hotels and casas which I have booked months in advance or at lunch the same day. If you are walking during high season, May – October you might want to reserve rather than take your chances.
I have done both, coastal and tradition inland and offer the following advice:
Porto to Caminha along the coast is beautiful and mostly flat along the coast. Portugal has built some relatively new boardwalks and bike paths parallel to the beach. It is very well marked. As you walk you pass through small beachside villages and larger towns. In just a few places one must go inland to avoid marches and in one case a golf course. This is great beginner walking, leisurely and very photogenic. I have done this twice and will again in May, from Porto to Vila du Conde.
Caminha to Vigo/Santiago along the coast after the Porto to Caminha portion you have a choice of routes, they are not all marked coming out of Caminha. 1.take the ferry across the Minho river to A Pasaxe, Spain and continue north along the coast. 2. go east along the river to the bridge (CG-4.2) and cross into O Pazo Spain. 3. walk east to Valença and cross into Tui, Spain. They all have value. It depends if you have had enough of gorgeous beaches! From Vigo you go inland to Rodondela and join the traditional inland route or take a variant to Pontevedra (no experience). The walk east along the river is also very nice, I recommend following the signs to the bike path in Valença rather than the marked Camino route which has a lot of hills.
Inland from Valença to Santiago The route from Valença to Santiago is beautiful and historic. The route is well marked and hilly. Give yourself time to enjoy the fort (Valença), and historic towns along the way. I particularly enjoyed Valeça, Pontevedra, and Padron. Highlights, treat yourself to the Parador in Pontevedra. and stop in Padron for the peppers by the same name. When you arrive in Santiago celebrate your accomplishment!
Inland from Santarem to Coimbra in my opinion, the pretty part of the Camino south of Porto. Take the train from Lisbon or you can park by the Santarem train station and begin following the arrows. Finding accommodation is a little more challenging on this route. Also, you may not see many other people, which is either a blessing or disappointing. As everywhere in Portugal it is safe, the food is good and people are friendly. I particularly enjoyed Tomar and the walk into the Roman ruins in Condeixa-a-nova (closed on Monday).