Quote - Ditto that, but don't worry about the purpose. 

I didn't start daily walks until I broke out of the need for a purpose and settled into the idea of simply enjoying the walk.  Previously I'd been walking to the store every 2nd or 3rd day, which had a purpose but wasn't enough.  DAILY is the critical variable for health and mood.

I agree. I used to walk 5k to work and 5k back. 5 days a week. It was good exercise. But as I got faster at walking, I got up later and later and had to walk faster and faster to get to work on time. Eventually, I ended up running. Then running faster. By the time I was running all the way to work, with backpack and all, I decided to take up road racing. Ran my first 8k and got a decent time.  

Then I got obsessed with running. Ran a half marathon. Ended up with a resting heart rate of less than 40 -- and a stress fracture. LOL.

The reason I like photography as an exercised hobby is the variation in exercise that it forces you to do. I did a lot of this as rehab. If you start photographing stuff that's a bit harder to get to, you have to stand on tip toe or crouch or whatever, and this is good for strengthening all sorts of muscles that you need later on, in old age, to help prevent falls. And it's good for younger people too, as my own running injuries made clear to me.

And if you want to make photography even more intense exercise, bring along a bunch of heavy equipment. :)

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.