The best answer would be to have another receptacle permanently installed in the wall. Any extension cord should be no longer than necessary and of the largest wire gauge possible. How many amps does the AC draw? What size breaker is on the circuit you are intending to plug into? For reference,

NEC

440.64 Supply Cords.

Where a flexible cord is used to supply a room air conditioner, the length of such cord shall not exceed 3.0 m (10 ft) for a nominal, 120-volt rating or 1.8 m (6 ft) for a nominal, 208or 240-volt rating.

3' extension cords labeled for AC units are available.

Don't plug it into a power strip...they often don't handle the high amps that an AC can draw. Either go with an extension cord whose specs will accommodate what your AC unit draws, or have the unit rewired properly, or have a receptacle installed that is closer to the AC unit.

Usually these wall units work on standard 15A lines, but I would get a 20A cord just for extra precaution. Code listed above says 10ft, hopefully there is an outlet within that. If the unit had a different plug in head then you would know it would require 20A wiring, outlet, and breaker. http://www.thinkecoinc.com/wp-content/uploads/2013/10/PlugReferenceGuide.pdf

Fred and Ken have the best answer. The code hates extension cords. Surface wiring as Ken suggests is good if you can't get into the wall, though going a couple of feet up vertically is a pretty trivial job in most finishes. A lot of times you don't even need to make any holes that need to be patched.

No patching necessary. Wire mold is the easy part. It just bangs into place. Decide what route your going to take. Decide what corners and turns you'll need along with how many ten foot sticks it'll take. Tools needed would be a hack saw, drill, and level. Not to mention a box of anchors with flat headed screws. I've be work where we actually used a wire mold bender for a slight offset. But mostly a project can be finished without a bender.

* I've worked when.

Wiremold works but it's butt ugly and frankly, unprofessional looking in most situations. I see you love the stuff as this is the second thread you've suggested it in.

I don't necessarily love the stuff. I agree it is rather ugly. That just what I might use in that situation speaking from if I were in the renters position.

Was an idea in that three too. I assure you I'm no wire mold salesman.

I want to make it clear I am only an apprentice who's been on the job since February of this year. My knowledge is rather limited at this point.