You certainly do have to look after yourself to stay healthy and fit in this industry.
When you first start to fly, it does seem very exahausting. This leads to just wanting to "flop out" on the lounge when you do finally get home.
But excercise, even if it is just going for a walk definatley gives you more energy for the next day.
If you can drag yourself to do something a little physical, you'll definatley start to feel alot better.
Otherwise you can easily just get stuck in the "couch potatoe" mode, and it becomes a bad routine!