Should I tell my boss I could be leaving? And should not be running?

Okay, this is a really strange subject. But, I am the athletic trainer in my company. Because of that, I am invited to go with the CEO across the ocean, and together with 2 other guys, run a sort of 10k company run. This takes place in the home town of our overseas mother company. Over there it is a tradition. Chances are pretty high that I will join that run in April, but I will start looking for a new job next year or so. So, in May or June, I will leave the company. They pay all the costs for the runners: About a week hotel, all together, flight, a week not productive. That's kinda mean, right?

You can always take a few days off next year and go and back and run with the previous people.

No the thing is, it's a bit of expensive trip. It's a sort of bonus, present, for my good work. I open the present, and then leave, and never come back. Well, I did really good work of course, otherwise they would not reward me with the bonus.

(Ah, it's a luxury problem nevertheless.. I should not complain.)

So it's a reward for your past performance: go for it! It's not like you promised to stay on to get it.

If they're rewarding you for past performance, then you've earned the trip. Anyway, you haven't resigned yet, and presumably when you do you'll be giving them fair notice, so I'd say until you're ready to leave, Keep On Running!