Some things I've learned over a lifetime of hiking:
1) Conditioning > 'fitness'. When you can move efficiently, you use so much less energy. Experience matters.
2) Strong hip muscles means fewer knee problems (especially downhill). Strong legs means less fatigue. Resistance training is key.
3) Pace: hiking in a group is fun, but don't burn yourself out keeping up. It's better for more capable people to walk slower than for slower people to walk fast. (And I like to move fast)