Although it's scientifically hard to prove, as far as I'm concerned dogs get excited before walks simply because they enjoy going out. As for the scent marking, dogs, as I'm sure you already know, have a very keen sense of smell (in wolves it's estimated to be 1000 times better than ours). This means not only are smells more apparent to them, but also that they are much better at identifying the individual constituents of any mixture of smells. So sniffing a scent marking is to a dog a highly interesting activity, considering they can tell such things as gender, age, diet, health, stress levels and of course, whether or not the other dog is ready to mate. I imagine that, if you had a sense of smell that good you would be much more prepared to sniff things. Or perhaps not.
Although the relevant behaviours are based on what was originally territorial marking it's probably best to think of them in this kind of situation as a calling card system, something like an olfactory e-mail, so that dogs can keep track of each other without necessarily meeting.