My personal thoughts (after now owning some VFFs for quite some time now) is that people need to spend 2 weeks walking in VFFs first, then keep runs below 1 mile for another 3 weeks before tackling a longer run.
Sprints are easy to do in VFFs, its how we sprint regardless of footwear. But runs...the pace along the ground slows from about 5 strides a second to 3 for most people. This causes the spring made by the ankle-knee-hip to contract more, and puts more pressure on the calves while rebounding, which doesn't happen in a normal shoe, and won't happen if you aren't POSE running in a normal shoe. It will happen in a VFF whether you like it or not, and if your musculature isn't ready for it, then you are going to feel it.
Another thing everyone should be doing are the 6 foot drills, which you can find here (wfs): http://download228.mediafire.com/bsz...oot+Drills.pdf