The first century I tried to do was on my 48 pound unloaded mtb. When my body shut down (pretty sure it was an asthma attack) after 50 miles my HRM said I was above 80% of max for a majority of the time. Simple reason - I had to work so hard to move that bike that my body was working twice as hard. The last 30 miles to get somewhere with a subway was the worst ride of my life, couldn't get above 10 mph without my chest cramping up again, any incline meant walking up.
Now it's harder to get my heart rate up to 80% on my road bike, but I've lost a lot more weight because I can ride for hours without killing myself. Plus the doc said don't go above 80% (can do damage and no benefits).
So, if you only have 30 mins to ride and going for longer rides is pretty much impossible I'd say get the heaviest bike you can, load it up with bricks and water and ride hard. Buddy of a buddy trains by dragging a car tire behind him on a mtb - but I'd say that's a much more intensive muscle building workout than purely cardio.