You can quit and they wont know but you will (Page 2)

And....on another note....triathlon is not the be all/end all of fitness.  There's a ton of good stuff out there.  Try kayaking, hiking, horseback riding, walking, weight lifting, yoga, find a climbing wall in your area, I could go on and on. 

In short, quit worrying about what you can't do and move on to something you can do if that's what it takes.

I remember your posts when you first expressed your goal to do an IM......it's not a new story, but it's also not a reason to quit the idea of getting fit.

Move on if you  have to......but MOVE.

Loads of good advice... agree, set achievable goals! I only wanted to add one thing: Find some stepping stones to your goal. 

I did a sprint and ran 10Ks before I ventured to the Oly. I won't do a HIM before I've done an Oly and run a half marathon, and I won't do an IM before I've done an HIM and a marathon. While I look in the calendar for races, I don't seriously consider signing up for anything beyond that next goal. I have another Oly, and a 10K this year. These are important repeats to get experience on those distances. Early next year I want to do a half marathon, and only then will I consider the HIM options later next year.

The run is particularly stressful to the body so just doing the run as a single event is a decent challenge that I think should achieved first before doing it as part of a tri. And stepping up, from 5K, 10K, Half marathon to marathon, you'll learn where your limits are. Giving more time you'll learn to handle those limits and work around them. Or maybe your doc was right to not recommend you run long distances, you'll know in time before risking permanent injury.

The IM might still be your goal ... but don't get too obsessed with it, you might find that the challenges on the way are much more fun.