Last night I did an hour of "Song In - Song Out" on the Kurt Kinetic Trainer. This is a favorite indoor bike trainer workout of mine.
Warmed up for 10 or so minutes and then started to ride hard for one song and then spin easy for the next song on my new MP3 player - and just keep repeating that for 60+ minutes. It's a good workout. I try and go max effort for the hard bits. Problem is you never know exactly how long the song is going to be - could be three minutes or it could be six! So you can get yourself in trouble and blow easy if not careful. Secret is finding the Edge, and not going over it and then surf along that edge for the duration of the song on the hard section. Pushing not bad gears - 53 x 18 or so, on the hard bits.
Finished up with a 10 minute mini core routine.