Now that I'm back in training mode, I'm completely convinced that to really get a serious cardio workout, it's senseless to try and do it if you run a commuting/city ratio (I'm at 49x18). Even on my bike with a 63" gear, my usual training grounds (Mt-Royal, a 6km light climb) are just too punishing to get any cardio at all - either I go slow and dragass it up, or have to hammer to go at the same speed I'd go on a geared bike. It's still fun, but my favorite gear is probably something like 36x20 with some variation on flatter/steeper bits.

I can still easily do it, and it's still fun, but I don't get to work on my breathing, pedal stroke, vary my heart rate, etc which is a lot more important to me than pure muscle-straining mashing. I have lots to say about why fixed training is senseless, but it's too much for a message board. The only advantage I can think of is doing the 6km decent downhill, which is great for working on extremely high rpms fixed and also for controlling speed with backpedaling (I usually go like a third of the speed I do on my geared bike on the fixed bikes on this hill, unless there's nobody out that day).

Long story short - if just being outside and getting exercise generally is the important thing, just use the bike you have. If you want to train train, use a geared bike unless you're in a very very flat place, and even then, I'd rather not personally.