"Base training" is a phrase that gets tossed around quite a bit, but it's important to pinpoint and understand what base training actually is, and what your goals and intentions are with it. The old school says lots of long, easy miles in the winter. "Ride lots," as Merckx is often credited as saying. But what good is being able to finish a 6-hour ride if you get dropped on the first climb of your first race, or if your longest race is only 3 hours, and most are an hour or less? Some riders will take the opposite approach. In their haste to be race-ready, and perhaps limited to the trainer in winter weather and limited daylight, they will flog themselves with high-intensity interval training, finding themselves flying once the season actually starts, but dying shortly after, burned out and tired by June.
Base training should focus primarily and almost entirely on aerobic energy systems, but that encompasses much more than just being able to do a 30-hour training week or a 6-hour endurance ride. Long riders are useful, and should be considered by those that have the time, but equally valuable are extensive and intensive sub-threshold intervals. A "base" interval may sound contradictory, but no matter how much time you have to train, they are crucial.
Your goals in a base training period are to increase your endurance and efficiency, and increase your functional threshold power. In a general sense, your goal is to increase the training load you can handle as way to improve those other factors. I generally summarize the main base period goal as increasing work capacity, that is, increasing the amount of training you can complete and recover from.
Some riders worry that endurance training is too easy. A rider wants to get better, so that rider goes out and trains as hard as they can whenever they can, with either extremely long easy rides, or extremely hard short rides. You can get pretty fit training that way, to a point, but the order you do those things in matters if you want to both be ready for the first races of the year, but also have your form last longer than four weeks. Riders should remember that training hard is actually quite easy, but training effectively takes self-control, restraint, and discipline. Training with 100% efforts all the time or letting yourself get sucked into racing on the group rides is an easy, obvious thing to do that is very rarely in line with an individual rider's specific needs.
The range for Easy is from 56% and 75% of functional threshold power, or 71-80% of threshold heart rate. This is the basic endurance zone, and the first aerobic capacity training zone. Anything easier than this is recovery, not training. Rides that focus primarily on this zone may range from 2 — 7 hours, depending on the background and needs of the rider. These rides are designed to burn fat and become more efficient at using fat as a fuel source, increases mitochondrial density, and increase blood plasma volume.
The range for Light is from 76% to 90% of functional threshold power, and 81-90% of threshold heart rate. Also known as extensive duration, or tempo. The emphasis here is still on endurance, although interval work in this zone will also have a positive effect on threshold power, and a smaller but still positive effect on VO2 Max. Fuel sources are the same as in as previous zones, though fat begins to be used less, and glycogen becomes the primary fuel source.
Interval lengths for this zone can vary from 15-60 minutes, with 15-180 minutes of total work, depending on the purpose of the intervals, and the ride in general. A straight "tempo ride" for the purpose of building endurance will typically consist of long interval blocks maximizing the time a rider has available for training. For newer riders early in a training program, that might be as little as 30-45 minutes, while seasoned pros might do 3 x 60 over the course of a 6+ hour ride. Expected physiological benefits from training in this zone include increased mitochondrial density, increased plasma volume, increased muscular endurance (time to fatigue), and increased muscle glycogen storage.
Also known as intensive duration, Middle is the specific threshold training zone. The range is between 91% and 105% of threshold power, and 91-100% of threshold heart rate. This power level, when normalized for weight, is probably the single most important determinant of ability to race effectively at any level. This is the system used when doing extended (>5 minute) climbs, time trials, and long breakaways. The more power one can produce at threshold, the "higher" all the other intensity levels shift as a result, since threshold power is the anchor point.
Interval lengths for Middle can be from 12-20 minutes, with 12-120 minutes of total work, again depending on the purpose of the intervals, ability of the athlete, and the ride in general. One way to think of threshold is as the most power you can continuously make for an hour. As such, it makes sense that a workout focusing specifically on building threshold power should include at least 15 minutes of work at this level, but probably no more than 60. Interval lengths when focusing on training threshold power ought to be in the 15-20 minute range. Shorter intervals can be used for opening up workouts, or as part of an intense warmup, but in general are not as effective at increasing threshold power as the longer blocks. The first 3-5 minutes of any interval in this zone are spent simply getting all metabolic systems up to speed and calibrating perceived exertion. That's perfect as a warmup, or as part of an opening ride, but less than ideal if the purpose is to get 30-60 minutes of real training in this zone.
Expected physiological benefits from training in this zone include increased mitochondrial density, increased plasma volume, improvement in VO2 Max, and increased sustainable power.
Notice that nowhere in these recommendations for the base period has there been mention of any anaerobic intervals. Save that work until your goal volumes of easy, light and middle have been achieved, and you're within 4 weeks of your first important events. Sprint workouts can still be included in the base period, ideally on the first day after a recovery day. With this approach, you should find the variable power and speed changes in your early races or group rides sting a little, but overall, the general pace will seem easier, your form will last longer, and the amount of high intensity intervals you can finish and recover from will increase. Normally in your base period, introducing racing or hard group rides once a week in the final month will provide you with all the supra-threshold work you will need to make that transition to racing and being fully in season.