As I read the OP's original post and ignore the rest of the thread - I get the impression that this is 95% vacation/relax/chat/girl time/whatever. I don't get the impression there is established business work going on. She says "photo mission" which I perceive to mean go walk around with a camera together and casually shoot with a purpose. However, there is a possibility that during this casual photo mission the chance to make some amazing athletic shots will occur which may improve the portfolio and impress the new friend from instagram.
So going with that perceived scenario - the OP needs to decide what she thinks will actually happen and weigh the value of the possible (but not definite?) important shoot. It seems like this is the 5% of the trip many contributors to the thread are focusing on with their posts (besides weather).
IMHO you have two choices...
1 - Take a ton of gear - everything you think will need for the 5% of your trip where you *might* get some amazing shots and improve your portfolio while impressing the instagram athlete friend.
2 - Take a light load - just what you will need to chill with your girlfriend for 95% of the trip and relax together while getting some nice snapshots during the trip and also an extra few items for the photo mission.
What part of the trip is most important to you? What is the true reason for the trip for you
Option 1 above means a lot of extra packing, expense for an extra bag, risk of damage/loss of the gear and maybe irritating the girlfriend that just wants to chill and not endure you keeping up with a lot of heavy photo gear. (You do have insurance on all your gear, right?) It might also mean you get a lot of great shots with the athlete but there's no guarantee. Are you sure this person won't cancel? There are lots of things that can mess up a shoot in any location including your own back yard. (Weather, sickness, change of plans, really bad light, etc.) So how much is this 5% of the trip worth to you to justify all the extra effort?
Option 2 means you relax, pack a lot less stuff and enjoy your holiday with your girlfriend without a lot of worry or stuff to deal with. Depending on the type of shots, could you just take a 5D3 and 2-3 lenses and call it a day? That will still require a backpack but it's a lot less and lighter than the 1DX, etc.
The other big factor is - how far are you travelling
? If this is a 2 HR DRIVE in your car, that's a LOT different, easier and less expensive than a 2 HR FLIGHT where all of that hassle and expense is involved. Because if this is a 2 hour drive, you could meet the athlete anytime with whatever gear you like just for a shoot with all the gear you want.
From your other posts, it seems like you are a seasoned traveller and are pretty experienced at dealing with all the gear. In case you're interested, one of my favorite pro photographers is Nevada Wier and she has perfected travelling over the decades and shares her tricks here... https://nevadawier.wordpress.com/
(Skim her blog for travel tips, etc.)
Let us know what you decide and then maybe you can share a pic or two with us after you've returned. Have fun!!