I have walked both the Senda Litoral and the Central out of Porto. I was 60 when I walked the coastal, 64 when walking the central - which I did from Lisbon. I enjoyed both routes, although they are very different, & I found both routes easy to walk, but I am fairly fit.
If you walk the coastal, or senda, I really recommend you walk out along the river. It's a beautiful walk, with only a small amount of industrial area.
If you take the central out of Porto, you have a lot of commercial, residential & some industrial.
This year on the central I walked the Espiritual Variante, which I loved. It has some steep hills, but they're manageable..
Both routes you're mostly on paths or trails, although there is some road walking. I don't find that difficult, as all my walking at home is on narrow rural roads with cars travelling at 80-100km/h.
Whichever route you choose, you're likely to find things you love, then others not so much.
Buen Camino
Jen