(Thanks for the additional information about your athletic background. You'll probably get better answers than you did on the TriNewbies.com bulletin board.)

Yes, you can definitely be ready for a half IM in 18 weeks. Provided that . . .

1. You are in reasonable fitness, which it sounds like your are,
2. You get get your swimming fitness up to where you can swim 2000 yards in the pool nonstop,
3. You practice open-water swimming once or twice,
4. You develop (or get) a proper training program (such programs are available online and in several books),
5. You get going on, and follow, the training program, and
6. You set a goal to just finish and you take each leg at a reasonable pace so that you don't get injured or run out of gas before the end.

Good luck. You'll do fine. The main thing is to stay confident in yourself and in your training program.

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Do not take counsel of your fears.--Andrew Jackson