My experience is yes. Doing 5 miles a day and some biking at the weekend and I'm pretty fit. It's certainly going to get you fitter than not commuting by bike.
Depends on what you ride a bit. I have definately chosen a bike for reliability rather than weight. Puncture proof tyres, 719 rims, cheap steal frame, generally cheap and heavy components means my road bike and mountain bike feel feather weight by comparrison.
I'n the year I've been doing a proper commute I started off on 42:16 and now comfortably ride 42:14.
Started to add a bit of interest as well doing different/longer routes home in summer and generally doing things like sprints on hills or timed runs to see if I'm getting quicker.