No matter what placement you have, without acoustic treatment you will never get the best experience. Remember, you can add very simple sound deadening for almost no cost - think soft furnishings, carpets, rugs and curtains. They will all help. We have a number of questions on acoustics so have a look for others there. Your problem is going to be that you are in a corner, and as you pointed out, in a long straight room - and bass loves corners. I'd suggest at least some form of acoustic trap in all four corners, but especially in the corner you sit at.
Okay, now positioning for the sub is less critical than you might expect. Yes, you want it close enough that you don't suffer latency/phase issues, but if it's in front of you, to the left or to the right won't really matter. The wavelength is long enough that you will not experience it as a directional sound.