How to ignore others?

You smile, say 'thank you for your concern,' and keep training.

^^ This!

People who don't understand what you're doing/going through and who only see the broadcasts of stuff like Kona will automatically jump to worst case scenario worry.  It's touching that they have that concern for you, so let them know that it's appreciated.   One of the things I like to do (not that a sprint triathlon for someone who has never done one is a cake walk) is ask people who have that reaction is ask them "can you swim?  could you bike for around 12-15 miles?  can you complete a 5K?"  If and when they answer yes to these questions I tell them "congratulations, you can do a sprint triathlon, and I'm happy to do one with you."  That tactic has actually gotten several family members and friends into this addiction.  My mother in law now does our local min-sprint mother's day triathlon along with my wife, and I am their sherpa and cheer squad.

Look within yourself.  

Do you *need* their input?  

If you think you do, then it will be hard to ignore what they're saying.  

If you don't, then you obviously are free to disregard what they're saying in whatever manner you choose.  What you choose will depend on how they otherwise are involved in your life.  For some of them, you can ignore them by no longer allowing them in your life.  For some, they'll be part of your life and you can either passively ignore them or actively train them about what comments you'll tolerate (or not).