If we are used to varied terrain and training on various routes that really give us a well-rounded aspect to our training, what do we do when we are indoors and logging time on a treadmill or turbo trainer? You can simulate the outdoors to a degree. You can, for example, use a harder gear on the bike to simulate a climb. You would sit back in the saddle and assume the seated body position you would when climbing, choose a gear that allows you to spin at 50-85rpm, and "grind it out". You could also mix up your indoor climbing with some out-of- the-saddle drills. Likewise, you could simulate a downhill by picking an easier gear than normal and spinning at a higher cadence, say 100+rpm. The treadmill is much easier for simulating hill work, in that you simply set the treadmill to Manual mode, and adjust the incline of the platform up or down.
If you are going to spend 40 minutes of an hour workout at 70-78% (for a solid aerobic session), you could easily mix that 40 minutes up between riding "on the flats", "climbing", and "descending" as you would outside. With the treadmill, you can program in hills of various steepness. And you're really climbing up! On the bike -- even with all of the "real life simulation" programs out there -- it's just not the same. You are not going up; you are not battling gravity. Instead, you are simply battling more resistance against the flywheel or more resistance that is self-imposed (by using a harder gear). That does not directly translate to climbing outside.
And, just to be clear, training indoors is worth the same amount of time as training outdoors. The body knows duration and intensity. So, a hour spent riding on the turbo trainer is worth ... an hour of riding. There are no shortcuts. Training indoors, especially with a volume-centric sport such as cycling, can certainly be a drain both mentally and emotionally. But, you've then got a choice to make based on how you're coming along in preparing to meet your goals. Maybe you've got enough time in order to shift things around within your training program. Or, maybe it's crunch time and, therefore, you simply need to knuckle down and put the time in.
In any case, mixing up your workouts should allow you to derive greater benefit from them because you are altering the way you use various muscles and muscle groups. Diversity is the key here, rather than specificity. By mixing it up, you will accomplish a couple of things: 1) You will help the time pass by more quickly and you will also help stem the tide of boredom and loathing that can quickly consume any indoor training regimen; and 2) You should squeeze more out of your indoor training than in the past.
All of this will help you enter the spring more refreshed mentally and stronger physically - both key elements to racing well come summer and beyond.