Think I've read in the past that 3 miles cycling = 1 mile running. In which case 20 minutes cycling is worth about 7 minutes running. If your goal is to use cycling as a cross training exercise for running then the equivilent of 7 minutes running simply isn't worth the bother.

On the few occasions when I've done sessions on a turbo I've pretty much copied what I'd do in track sessions. 12 x 1/4 mile with 1 min recovery, 6 x 1/2 mile etc. Basically 5k of efforts with a good 20min warm up & warm down. I've found sessions such as above more enjoyable due to the variety, than just cranking out miles.