first, ride a GOOD bike. it doesn't matter how much it cost, you could spend a little or a lot for a good bike. I mean, why ride a bike that you're not happy with, it doesn't matter if you're doing a weekend ride or commuting, a ride is a ride and you should have a good bike (fit, comfortable, good handling, fast, and durable...not in any order). Sure, if you have certain needs for a bike (rack to carry pack, fenders, wider tires, etc.) you should still ride a bike that you love to ride.
I have a track bike and find that the work out on the flats is less than a road bike. Thats because you have a top speed you can maintain and your legs learn to spin. Spinning is good and it is a workout, but on a long flat road, spinning is easy. But climbing is more of a workout. And decending is a workout. In San Francisco, a fix gear is a great workout.
here is an interesting website:
http://www.nycbikes.com/index.php
they have a fixie for $750, but they have a cross for $550! I don't get it, if they can build a cross bike for 550, why can't they build a fixie for that price? less parts should be less money!