When I started this over 10 years ago, I jumped from Sprint to Ironman in a year so I think I'm not one to judge. However, that said, I think the more time you spend on the shorter distances, the more you will be prepared for the longer distances. Think about anything we do in life. The longer you do it, the more proficient you become through the knowledge you gain along the way.

I think I would do multiple races each year. This year, do a few sprints, next year, do a few sprints and an Olympic, the following year, sprints, Olympics and a half, the following year Halfs and Full. Or something along those lines.

You will get a better understanding of racing itself, transition, nutrition intake, bonking, the mental game, the mass swim start and everything else I can't think of.

Good on you for wanting to make a life change and get off the couch. That, my friend, was the hardest part.
Your goals are very realistic and quite do-able...in fact you could probably race a little more. Make sure you get out for some road races (running) and some organized bike rides. Look for a local tri club, or at least a local running club.
A first IM at 40? No problem. I've met First time IMers in their 60's.
Best of luck.

I'm planning on the same buildup as you, I think its a rather natural progression if you are trying to increase endurance by making your goal to complete a longer race each year.

I did my first sprint tri in 2006, in 2007 I did two sprints and an oly. In 2008 I have registered for a HIM, an oly and a sprint so far....I plan on adding some more B priority sprints where ever they fit in. Then in 2009 I will be graduating college and I will hopefully celebrate by hearing "YOU are an IRONMAN".

Good luck and stay motivated! Remember that motivation is the key to consistency, and consistency with your workouts will help you to improve.

This year is my first season and I'm planning on going Sprint-Olympic. Next year I'm hoping to go Oly-half. Maybe after that I'll go for the full, but we'll see how the previous seasons and life pan out; but I want to get that full 140.6 in before I'm 30.

I was in your boat last year at 36. I started training in January, did my first sprint in May, a few more sprints and 2 OLYs before finishing the Steelhead half in August. I did not set land speed records but I needed to 'set the hook' well so I'd stick with it - plus I like competition even if I wasn't really competitive. I followed a 12 week program unfit to oly program and then a transitioned to a 20 week online 1/2 iron program when I was 20 weeks out.

It was probably a bit much as I did need a week off to get my sleep back ( a three year old and a 8 month pregnant wife added to that - they were great cheerleaders though) but again it wasn't for speed it was for finishing.

i'd be glad to share more if you want to know more about how I'm adjusting my plan this year as I am planning roughly the same calendar.

good luck. I'm eyeing afull ironman for 40 too.

Thanks to everyone for the support. I'm psych'd about doing this. I started daily training and eating healthy, the day after Christmas. "Getting healthy" was a Christmas gift to me, plus I just hate New Years resolution's. So before that, I pretty much ate crap, and too much of it at that. And I was lucky if I got to walk or bike once or maybe twice a week.

So anyway, in one month I've lost 11#, and I feel great. I'd like to lose another 13# to put me at my target of 175#. I'm committed to keeping my training in the aerobic zone (I try not to let the HRM go over 146) for at least another two months, then I'll start to hit it a little harder.

Wish me luck! :)