I walked a 12k with my dad a couple of years ago (this year I jogged/ran), and it was great... but we were amidst 20,000 other people, half of whom were also walking (and anyone jogging was in front of us anyway). I guess we jogged a little, but not much.

Without someone there you can talk to and pair with and any sense of atmosphere... I'd go nuts walking that far, too.

I guess it comes down to: would the need to walk for that long (with maybe a little jogging if you can muster it) be an acceptable price to pay for the benefit of getting to swim and ride longer?

Tough choice.