Don't try to be healthy, try to be strong or try to do an ultra - or even crossfit. Community & nutrition usually follow IMO.
The number one reason I see people fail their diet/exercise goals is because they go to big to fast.
Training to do an ultra marathon if youve never run a 5k, swam a mile in open water, or even mountain biked for 5 miles is a sure fire way to get injured. You need to build a base of knowledge, muscle, and nutritional habits or you set yourself up for failure, imo. Not a hard rule, but close.
Crossfit is the last thing I would recommend to anyone, especially a beginner.
I didn't say anything about doing it fast. Prepping for an ultra is a long term thing.
I think that setting goals like "do an ultra" is more likely to have people stop entirely rather than downgrading their goal to something more doable when they hit that first roadblock.
Better to start small and catch the running bug than try to do something that the vast majority of the population will never even aspire to.
You have to remember that something like 40% of Americans are obese.
You're right, a 5k is pretty doable, but if you get to 5k and you don't find yourself enjoying it, you're never going to get to an ultra, but if you do find yourself enjoying it, it's much easier to work your way up to a 10k. And there's a lot of points between 5k and ultra where the game changes - a 10k feels different and has a different challenge than a half which feels different than a full (I can't speak for an ultra), and many of those points are perfectly healthy to stall at.
Long term goals are great, but only once you've set yourself on that path. The goal of running for a month and a half is enough if you've never run before.
Also, ultras will overwork the shit out of your heart. (Bro science)