Oh, and one more thing. If there's a midpoint somewhat accessible, there are two-port PoE powered switches you can drop in the middle of the span; some of them even will chain PoE; I think i've seen something that would let you put in three of the chainable switches between the power source and your endpoint (you could probably chain a couple more if the endpoint is also a powered switch).

It would certainly make network engineers cry, but you have to work with the site and wiring you have, and only sometimes is it easy or convenient to run new cabling.