It's not cool to refer to your fixed gear bicycle as a fixie. I do it all the time. I'm so uncool.

Your ass will harden up given time, but you might need to work on your positioning to get ovr the neck soreness. Starting with everything set as it is, move around on the bike and see if you can get more comfortable. if you still feel too stretched out, try sliding the saddle forward in small increments, rotating the bars a little. You may need a shorter stem or you may need to raise the bars up a bit, but give yourself time to adjust to new positioning before you make another adjustment, and start with small adjustments first. You will limber up over time though, and the core muscles will strengthen up so less of your weight is supported by your arms. Maybe that's all you need.