So, wait, why has everyone aged in the returned timeline and yet Brand apparently hasn't? Recall, Brand received a message from Murph informing her her dad had died -- and Murph was, what? in her 40s at that point. Event with the blackhole happened shortly after that. So, by extension, Brand would be significantly older or dead when Cooper decided to go find her at the end.
I believe the slingshot around the black hole caused the majority of the 80 year time dilation. the movie plays a little bit too fast and loose with its time dilation rules, like being on the water planet caused 23 years of time dilation (for 2-3 hours spent on the planet) and that was probably 99.5% close to the black hole, slingshotting off the black hole should really have caused something like 100 million years (or more) of time dilation for brand and tex since that was like 99.9999% close to the black hole -- far far far far more than 80 years in any case.
But we just wave our hands and say 80 years passed for both Brand and Tex. Then Brand spends another 1-2 years in hibernation while her ship travels and lands on her dead lovers planet, she buries his body and starts her own little colony.
Meanwhile Tex enters the black hole and magic happens where he enters some sort of tessaract construct, is able to jump around time including the past, and exits the black hole at more or less the exact second he entered (80 years having already passed due to the slingshot around black hole) but exits at the wormhole around saturn.
Actually we don't really know when exactly he exited, it could have added anywhere from 0 to a few years after his entrance into the black hole, so in theory he could beat Brand to her destination or could arrive at the exact same second she lands. Hell, if the future humans were a bit more benevolent they could have had him exit -10 years from his entrance point and he could have saved everyone a lot of hassle, but clearly they are huge assholes. Time travel is funky.
It actually would have been a pretty awesome ending if 1 billion years had passed while they did the slingshot, and when they come out humanity has spread out everywhere in the galaxy thanks to Tex's morse code message and they are welcomed into the new "crazy ass future humans" with open arms. literally open arms, since most humans now have 8 arms and 4 heads.