Gretchen Reynolds, who wrote the excellent book The First 20 Minutes, wrote a good round-up post on her blog on the NYT website.
For Fitness, Intensity Matters
You won't find much direct advice in there, but a lot of links to articles she's written on the subject.
The TLDR version of all of this is, you need to work out hard if you aren't going to work out long, and if you work out hard you don't need to work out all that long. This mostly applies to general "getting in shape" and untrained people, but it's worth trying out for anyone.