ok over 50 with big goals advice please

First off, congratulations on deciding to become more healthy and pursue a more active lifestyle.

I worked with an athlete that went from "Marathon" to "Ironman" in well under a year.  While he had marathon fitness he had ZERO experience swimming (aside from recreational swimming) or cycling.  So it CAN be done.  That it can be done however is not necessarily the same as it SHOULD be done.  You aren't coming from "marathon fitness" which complicates the calculus.  The athlete I worked with was HIGHLY motivated - as evidenced by him investing several-thousand dollars early in his training ($10k+ for a new road bike and a triathlon bike, power meters for both bikes, Garmin Fenix5, Edge 520, plus other miscellaneous gear and a bike fit with with one of the most respected bike fitters in the United States).  I'm not for a moment implying you aren't equally motivated, nor that you must validate your motivation with a large money expenditure.  I'm merely saying when I started working with him, there was tangible evidence of his motivation to do the training necessary to achieve his goal.

My question to you is: WHY do you want to do an Ironman in a year?  Why is that more compelling than having a reasonable plan to get you to an Ironman in two or even three years?  By your own admission, you've been off and on for several years.  To go from "mostly off" to a training schedule of 15-20 hours/week is a HUGE change.  Granted it would be several months before you got to 15-20 hours/week but consistency in training is the foundation that an Ironman training plan is built upon.  Setting yourself up for success by having a planned build over a couple of years is a reasonable approach.  You're in Texas so the race season goes later into the fall than other areas of the country.  You can easily get in a sprint or two, maybe even a late season Olympic this year.  Take the off season to continue building your aerobic engine, then next season start with a couple Olympics and step up to a mid/late season 70.3, perhaps even two.  Then, take on an Ironman the following season.  You'd be in a MUCH better position to a) enjoy the Ironman, and b) do well in the Ironman as opposed to being under-trained and struggling to finish the Ironman.

If you are absolutely intent on doing an Ironman in a year, yes, you can likely accomplish that goal.  I tell my athletes all the time that a successful Ironman isn't about fitness - assuming you have the base level of fitness to complete the distance.  It takes time to learn the things you need to know, and be skilled at, in order to have a successful race - specifically pacing, nutrition/hydration, the mental aspect, transitions; not to mention swim/bike/run.  To do it in a year, everything will have to go right - very few missed workouts, no injuries, life not interfering with your plans, etc.  Overweight and over 50 doesn't line up well with no injuries when you consider the build you'd need to do to complete an Ironman in a year (a 2-3 year plan would greatly reduce the likelihood of injury).   If you can accomplish all that, and are willing to accept a VERY long and possibly painful race day, then go for it.

But again I ask WHY?

Welcome back!  

I mentored a guy who went from couch to IMKY a few years back (so he completed his IM in August).  

Your challenge is likely to be impatience, and not being where you once were and therefore pushing too hard for gains and ending up injured.  

It typically takes about three years to peak in endurance sports. 

You don't have to wait three years to do your first race, but you shouldn't expect to be in peak condition in one year and if you are racing in your first three years it should be to gain experience, measure progress, and to maintain motivation, focus, and direction. 

Having said that My first triathlon was a 70.3.  I did the 1-mile to 70.3 in 8 months.  I had been a runner for 30 years but it had been off-and-on the four years before starting my training for the 70.3 with more off than on.  I did an Ultra Ragnar Realy 10 months before starting my training for the 70.3 and had not done enough running since the Ragna Relay to really count at all.  I was on the 1-mile starting point. 

My first step was to lay out an 8-week 5K plan to gain some fitness back.  Since I had zero backgrounds with swimming and cycling I held off on those until I had some running base back.  I did the 5K 8 weeks later and it wasn't a really pretty performance but I was making progress.  I then did an 8 week half marathon training block.  By the time I got to the half marathon I was feeling like I was in shape.  I ran a 1 hr 30 minute half marathon which was better than the 1hr 36 minutes I had done in the same race the year before on 4 weeks of training (the more off than on plan) but nowhere near the sub 1hr 23 minute PR I had set four years earlier at a time when I had been consistent in my running for 2+ years. 

After the half Marathon, I started looking for a pool and a bike and two weeks later started an 8-week build training block that included swimming, cycling, and running.  After that, I did an 8-week race-specific training block to get me ready for the 70.3. 

I made huge improvements.  My bike pace on training rides increase from 18.5 MPH averaged to 19.9 MPH averages and the swimming improvements were twice that.  My running also improved.  I wasn't anywhere close to my 1 hr 23 minute half marathon pace but I and sure I could have done a 1 hr 26 minute open half marathon.  

On race day I was in the best shape of my life.  As a competitive runner, I would put in about 5-6 hours of training a week.  For the 70.3 triathlons, I had built up to almost 10 hours a week.   I felt that my potential was going to be about a 5 hours race time.  Well...my race time was over 7 hours.  I was NOT to my peak year despite being consistent in training for 8-1/2 months.  I, however, was thrilled to have finished the race and couldn't have been more proud of myself if I had been first place overall. 

I found a new drive in a new sport and so after the race I stuck with triathlon.  I have been doing Triathlons for four years now and finally went under 5 hours in the 70.3 distance last year  I don't know if I have peaked yet because I have done 8 triathlons and have set 8 PR's. 

I have never done a 140.6.  I don't have a desire to do one (yet) and mostly felt that there was no point in doing a 140.6 race as long as a 70.3 was holding my attention.  I figured that doing a 140.6 too early in my triathlon career would hurt my chances for success and possibly make me lose my drive for the sport to where I would slip into "more time off than on" like I had done with my running before finding triathlons. 

Yes, you can get ready for a race in a year.  Your results may be like my results were with going from zero to 70.3 in 8 months and finding that even after getting in the best shape of my life was still 2 hours over my potential at the half Iron distance. 

While I didn't do well in my 70.3 after 8 months It was a thrill to finish a race like that and it left me knowing that if I stuck with it for 3-4 years that I would see where my potential in the sport could be.  If you do the full IM in a year expect to do it for the experience and to build your motivation.  Don't expect to perform at your potential though. That will take a few years to get to when you are starting at one mile.   

I am all about goals...but wondering...when you were 'on and off' triathlon- whats the distance? are we talking sprints & Olympics or longer / have you done marathons? and are you fit at all as you start or are you starting from the couch?  an ambitious goal in my mind would be to start tomorrow committing to beginning a plan for a sprint sign up!  and a consistent block of training- with the goal of signing up for a carefully selected late season full next year...just my 2 cents