I don’t think this is an issue nowadays op because, same as pulpJarvis, you can buy so many cross-over types of trousers that are half way between a nicely cut yoga pant, and a pair of trousers you can wear to the office, that are stretchy and have a grown on or “false” waistband.
Marks and Spencer do a couple of versions in black and navy, and probably olive and brown or maroon this season too. Try a few on to get the best fit and length. Some have pleating at the front and others are flat fronted.
Saint and Sofia do a slightly more upmarket version in various lengths.
They don’t necessarily look like leisure wear because the fabric is slightly heavier than that used for s legging, it hangs better and doesn’t crease, and you can dress them up or down, but they have the comfort factor.
Layers on top are also useful because it can be boiling on a plane one minute and freezing the next. I usually wear a long sleeve cotton mix ribbed t-shirt from H&M or a thin cotton polo shirt with a collar and some sort of cardigan or thin merino wool jumper on top and have a rain jacket that isn’t too bulky for outside.
I always think that shoes you can run in if necessary and long thin cotton or wool socks are a sensible option, so I wear my heaviest shoes such as moccasins or loafers, so I don’t have to put them in my suitcase, or a pair of smart looking trainers, or trainers x shoe hybrid, if I am doing lots of walking at my destination. And I often have a thin wool folded pashmina type scarf with me that folds up in my carry-on bag that serves as a blanket or pillow.
You can dress the above outfit up to look smart or as casual as you like. I see lots of women wearing similar outfits. I have even been upgraded a couple of times wearing it. But I don’t think many people are worried about what others wear tbh as long as you are reasonably presentable, especially late in the evening, when comfort and warmth are a priority.
I think there’s more judgement about constantly wearing leisure wear for daily life, when not travelling, outside of the gym I mean. That definitely exists in some EU counties, particularly France, but the UK is much more forgiving!