Sulu,
I walked the Interior in September. I started in Viseu and traveled 18 days to SdC - >250 miles (>400K). It was, without a doubt, quite beautiful. It was the season when the grapes were full, flowers were blooming and even the pumpkins were big! I had several days of rain and often the way was a bit wet, muddy, and full of water. The way is quite difficult; you are in and out of river valleys (wine land!). I traveled the Frances' in '09 and have commented this was half as far and twice as hard. There was an occasional time when the way was not marked well, but normally quite well marked. It parallels major highways and as Arelio Simones rightly guided me, when you get a bit mis-oriented focus on the roadways. As the way joins La Plata, it becomes much more crowded. I traveled alone and was often the only one in an overnight location - sometimes no one had been there for more than a week. I met some of the finest folks you could imagine and everyone was helpful to this solitary American Pilgrim! I speak almost no Portuguese and some Spanish, but many spoke English and were quite helpful. I commend to you the wine! I've promised several that I'd post / provide reflections on my journey, but I've just found myself too busy to put them all together [I retired the day I left the USA for the journey and have been trying to get the things done I promised I would do after I retired!] I would commend the Interior route to you - but I would not recommend it as the first one that a person tries or if they travel alone without some experience. If you / others have other questions I'll be glad to answer them from my perspective.
By the way, if you travel through Lisbon please connect with Arelio and / or Diogo who visit this forum and are very fine guides and hosts. Part of the treat was meeting Ivar. He was very helpful in SdC!
Bon Caminho
Rich Cooper
flewitheagles