Accidentally Green

I've been having some car trouble of late... which is nothing new, although this looks to be an expensive fix. In the mean time, I'm working part time about 20 miles from home, a few days a week. So I'm commuting by bicycle.
I didn't exactly plan it, but I started on Saturday, and will be bicycle commuting at least for this week, which happens to be "Earth Week" (Earth Day is Wednesday). I've always tried to be at least somewhat environmentally conscious, I drink from re-usable bottles, try to limit my trash, turn off lights, etc., for the time being I'm driving a lot less, too. I expect to save around 160 miles of driving over the course of this week by bicycling. Not too shabby, I suppose.
I rode Saturday on my fixed gear, which is, I was reminded, not ideal for extended commutes with a backpack. The handlebars are too low and it's hard work to keep going over rolling hills. It's a lot of fun for short (under 20 miles total) faster rides, but for slogging to work and back... I'm not in good enough shape.
It turns out running keeps your legs in shape, they were not too tired, but my hands, shoulders back and bum were a little upset with me.
So I fixed up my dad's old Schwinn hybrid with skinny road tires, new shifters, fenders and a bell (what's the fun of a commuter bike without a bell, I ask you?) and will be trying that out tomorrow. I'm hoping between the wide gearing range, upright riding position and rack to stow my stuff, rather than a backpack or messenger bag, I'll have a more relaxed commute, although I'll still be burning some decent calories.
I'll get a pic of the new commuter rig up soon. With the rack and fenders it's got a bit of Euro style to it. I'm looking forward to it.
A good question, though, is that if an active-but-chubby guy like myself can manage a 40 mile round trip to work on a bicycle, what can the rest of us do?