Hi Steely Dan,
In my experience (no scientific data or studies, sry!) one of the major reasons that commuting by bike is so healthy is because it's twice-a-day, and as long as you get in a good twenty minutes each direction the extra miles only give granular improvements in fitness. If I look through my bike logs (I'm 61 and have kept detailed logs since 1990) I discover that my fitness level has always been better when commuting short distances than riding a long single ride daily, and the fitness difference is not that much different than commuting longer distances.
Of course I'm talking about overall fitness and health; if you're interested in training or increasing endurance or other bike-related goals, yes you need the distance. But for general fitness and health (and bikehappiness) I don't think you will notice much difference with the shorter commute.
Again, I'm just speaking from my personal experience.
