Are you saying a half marathon or half ironman? I would say a half marathon is certainly feasible for most people in the time frame you mention. A half ironman is entirely different. You didn't mention your biking or swimming experience. Not that it isn't possible, but the running is MUCH harder in triathlon because of biking. I've only done sprint triathlons, and the run is only 3 miles, but it doesn't feel like 3 miles because the biking is rough on the legs. My personal opinion is that you should look for a sprint or olympic triathlon before trying a half ironman. I suspect either a sprint or olympic triathlon would give you the exposure to triathlons that could let you decide whether you'd like to try to move up to a half ironman.

I agree and will go even further and suggest you start with a sprint triathlon before moving up to the olympic distance.