I wear my hair in a ponytail and try to get it all into the tube scarf at the back, but it always manages to get out with shoulder-checks and other movement. It ends up matting so quickly, I was wondering if there were any simple hacks I was missing.

I've been either adding an additional tie or two, with one tighter and closer to the end (but it doesn't always stay) or more often I use a long "Buff" (the dominant brand for the fabric tube "multifunctional neck gater" products) worn like a bandana and don't bother with a tie. For my riding at least, if the buff is as long as or longer than my hair, I usually don't end up with the tangles I'd normally get. If I don't want it flapping in the wind, it is also long enough to tuck the buff's end into my collar (or just put my jacket on over top of the buff and pull out a little slack) and it stays there even with shoulder-checks.

It might look similar to the high tail thing someone else suggested so maybe my suggestion isn't too helpful.