This is perfect for the gym, and works even though I have bangs :)
Start by bending forwards (keeping your head upside down), and make a French Braid starting from the neck. When I got on top of the head, I took the hair from the front into the mix in three sections so the hair wouldn't start pulling downwards. Then just secure with hairband. If you like, you can make the ponytail into a bun as well.
(Buns aren't too glamorous with this fine Finnish hair, but you get the point)