I'll start off by saying that I'm not completely new to running, I've raced 5k and 1/2 marathons and have run marathon+ distances. I'm just out of shape. "6 or 8 month's of sitting in front of the computer working long hours" out of shape.
When i stopped running I was in "casual" training for my first ultra, but a reoccuring IT problem mixed with a new career and a lot of traveling got in the way. Now that spring is around the corner I want to get back into training for my first ultra. I would like to choose a specific race to shoot for in order to keep myself motivated, but I'm not sure how much time I should give myself to train. I know the usual route is to train for a 5k, then move up to a 1/2 marathon training plan, and keep moving up through the distances, but I've already done those aand would rather just shoot for the race distance I want to run.
It took me about 36 weeks to get me to get to a reasonable marathon pace, so I'm thinking maybe 45 for a 50k.
Everything depends upon how fast you want to run and how many miles you log between now and then; that said, a 50K really isn't all that much further than a full marathon. Last April I started running again after a "six to eight month layoff sitting in front of a computer for long hours", and effectively went from the couch to running 33 miles (split into six legs) in last September's 15th installment of Reach the Beach - New Hampshire, and by virtue of the fact that I had concentrated solely on "Long Slow Distance" (aka. LSD), I really had no problem with the event (I actually got faster as the legs and miles wore on).
Were I coaching you, I would encourage you to ramp your mileage up into the 200+ mile per month realm, and after you've been able to do that for at least three months, I'd say running the 50K would be fairly easy. If you can hit the 200+ threshold six months before your planned event, so much the better, as you'll be even faster.
First you need to build a good base of 20-25 miles per week. If you are truly starting from the couch (or the desk chair), that could take 4 or 5 months, avoiding injury. Once you're at that level for a few weeks, pick (or build your own) a good training plan for the 50K. That would probably be about 18 to 24 weeks. As shipo said, 50K is not much longer than a marathon, so adding a few weeks and some longer runs should get you there. Good luck.