IM Training Plan for Slow Person

A 6-hour ride should get you pretty close to 100 miles (you can expect to get a little faster with focused, consistent, training).  A 3-hour run will be 15-17 miles at the paces you're quoting.  You can do OK at an IM with those kinds of numbers.  There is a psychological advantage to covering 112 miles in a training ride (and also a risk), but not usually a good reason to punish your body with a 26-mile training run (although you could do a stand-alone marathon for the experience). 

I would go by distance on the bike since you will be on the course longer.  On the run I'd stick to the time.

I think personally, with the IM that far out, I would do a Bike / Swim focus and just hold serve on the run.  Probably a 4/4/2 ratio for the S/B/R, with no long runs untill about 4 months out.  

The reason is at your weight the Swim and Bike are the lowest impact, and running gives you the greatest chance of injury.  At about 6 months out I would balance it out more, but right now you want to really focus on low impact, high calorie exercises and try to drop some pounds and still build a decent base. 

It sounds like going by the time is fine for you.  I share concerns for longer runs, but hopefully you are going to drop 20 pounds or so by the time you are into the 13 + mile runs.  It is a journey.  Welcome to it.