surprised no one mentioned this: don't pay too much attention to "miles" on a trainer. friction/load effects don't match the real world - you're not actually cutting thru the air with resistance, nor do you have any forward momentum. not better/worse, just different. I can easily tweak the outcome of miles based on resistance and gearing. and my rollers versus my magne-trainer also have different characteristics. speed and distance are relative only to the specific device and bike.
instead, simply figure out how much time you want to budget for this and dedicate yourself to meeting that as a daily or weekly goal.
focus on your effort for a particular workout, or even better, use a heart-rate monitor. load your days differently - sometimes a steady-state, sometimes a tempo, sometimes a recovery, sometimes high intensity intervals (HIIT). this variety is important, and you'll learn to "know" what your workout should be for any given day. I tend to take the fifth or sixth day off as full rest.
and it pays to cross-train with other exercises that build both core and other muscle sets. swimming and rowing form the "triumvirate" IMO. This really does improve your capabilities on the bike. I have a rower set up next to the bike trainer. the too-damn-early alarm goes off every day and I've trained myself to say "don't think." put on the right shorts and go to the basement. bike one day, rower the next. my constraint is about spare time, so that's what I budget.