I'm getting a lot of fitness out of 45 minutes of trainer tempo in the morning, followed by 45-75 minutes of road tempo at "lunch." When I start doing intervals again, I'm planning to continue the tempo in the mornings (3-4x/wk), and do all the intervals on the lunch rides. I've done this in the past with two 20-25 minute commutes at tempo, and intervals at lunch. That bounced me the fitness I needed to win a bunch of races and upgrade to Cat 2, and as soon as I lost the commutes, I had to downgrade...
Split workouts can be gems.