I'm by no means an expert mechanic, but I do usually replace my own cables. My feeling is to replace the cable housings periodically--although what 'period', I cannot say. On my racing bike, the shifting has been smooth, so I have left those alone but, on the rear brake, it was getting sticky and I couldn't fix it, took it to my LBS (local bike store). The mechanic determined that the housings were getting mucked up inside and replaced them, problem solved. This was on a racing bike with about 20,000 miles on it in almost exclusively dry conditions so, if you ride in the wet, the housings will no doubt get gucky much faster.
Regarding the internal routing, I have no experience with that. All I can suggest is that you (a) try doing a web search on it and (b) try to develop a relationship with an ace mechanic at an LBS (minimum 5 years full-time experience) and ask them. What I do is go in, buy something and then ask, so I'm not trying to 'mooch'...
P.S. I can't say for sure the derailleur cables are the originals from 20,000 miles ago. I may have replaced them at some point.