i try and do intervals twice a week (usually ends up being once a week!!) in the gym. for me this consists of 2 x 20 minute intervals at just over 85% of my max heart rate (MHR). I throw in a 15 minute warm up before and after and 5 minute recovery all at 65% MHR.
as mentioned above try and mix it up to prevent boredom. if you are tired, just go in and cruise. try an interval session like the one above once a week or maybe try and ride for 60mins at a steady quick pace, or try some of the "hill programs" on the gym bike.
I can't ride inside without my music though....!