New to Triathlons, but not doing so well the training

I quit smoking about 4 years ago, and started training for a marathon; I trained for about 6 months and ran the marathon. I have run several marathons and mini-marathons over the last 3 years. Then, I did a speech on an Iron Man Challenge about 3 years ago in my freshman year of college (I started college late.) I am now 32 years old, said "I've have had enough of the running, and I'm going to train for a Iron Man." I started this week and I am making most of the training, having a hard time with the swim, and keeping in one HR zone. The biking  and the running is is going just fine, I'm reading a lot of the articles and learning a lot about the nutrition. Will the swimming just hit me all at once? Then I get the  right stroke, or what is a good way to learn this. I push my self every time I swim, and I like to swim, I just am unable to swim more than 100 yards with out stopping. 

Thanks,

James