We're set up with Computrainers on computer courses on our trainers. They do some hocus-pocus with a calibrator, including factoring our weight, and then it's ready set GO! on the virtual course, which for us is 10K.
There are graphics of our bikes and a display of speed, power, and distance done, as well as distance between the riders. You get to totally focus on yourself and the numbers without driving into trees or anything :-) Utter klutzes such as myself like that; the folks who really like racing probably hate it. Aerobars are useless too - fans are very helpful.
The stuff about hovering right at that 'lactate threshold' is what it's all about... if only I knew where mine was...where did I put it last?