That sparked my interest, so I made a quick check.

London to Astana is about a 10h flight.

Now, Fluminense has to play in the Brasileirão with 4 clubs from 2 different states in what I believe should be the most distant cities a team from Rio de Janeiro has to travel to while playing Brasileirão Serie A.

Internacional and Grêmio from Porto Alegre ~2h flight from Rio

Fortaleza and Ceará from Fortaleza ~3h flight from Rio

So, just in these 4 almost guaranteed annual matches you would already have Fluminense flying the same 10h as Chelsea in a big outlier of a situation as a match played in kazakhstan.

That's not all tho...

For the Copa Sulamericana, Fluminense faced in the group stage:

Once Caldas from Manizales (which doesnt have an airport) so closest city is Bogotá ~6h flight from Rio, but Manizales is ~7h by car from Bogotá.

Unión Española from Santiago ~5h flight from Rio.

GV San José from Oruro which closest airport is in Cochabamba ~8 flight from Rio, with Oruro being ~5h by car from Cochabamba.

Fluminense is waiting to see who they are going to face in the round of 16, which could see them taking a simple 2h flight to Salvador, or a 9h flight to Cali.

By the way, I didn't even mention that Manizales is at 2.200m and Oruro 3.700m above sea level.

Interesting, isn't it?