You want to look at the progressions of macro and microcycles in training. Microcycles are usually week-long and have certain numbers of rest and workout days. Macrocycles are a series of microcycles grouped together and lasts about 6-10 weeks. You can consider the start & end of a macrocycle as a rest-week, low-mileage and low-intensity.
When doing intervals, they are "instead of" some other workout, typically a longer endurance ride. So a "endurance-week" in a macrocycle may be have 400-500 miles with two rides over 100-miles. There won't be any day where you're doing sprints and intervals in an endurance-week. Then there are "intense-weeks" where you do one day of sprints and one day of intervals. Total mileage for this week might be only 100-miles max.
In regards to Ricardo's question about the base, it's really just to work on form and have basic joint & ligament strength down. When you start pushing at 95-100% efforts, this puts A LOT of force and stress on your joints and ligaments. Having smooth and efficient form (pedaling motions) down is essential to prevent injuries.
I'm sure you've followed some weekend-warriors down the bike-path. If you look carefully at a spot on the rear of their knee, you'll see that the knee moves laterally outwards and inwards in a figure-8 pattern? This is caused by the sequential activation of the 10-12 muscles involved in the cycling motion. But rapid on-off transitions can cause wobbling of the knee. So if you're smooth, the forces are more even through out the 360-degree pedal-stroke and you put less stress on your joints. Watching someone who's smooth, like a pro-racer and you'll see that there's very little wobbling of their legs or their upper-body; more of the muscle-forces are in the plane of rotation of the crank and driving the bike forwards.
So.. after you've got your form down, then work on the intervals. Actually, I recommend doing tempo workouts for a couple weeks before intervals. Here's a basic intro macrocycle for beginners:
week 1-3: Endurance & base. Do 1-3 hour rides of 20-70 miles at a time, steady pace the whole time. For endurance, pick up the pace to something you can hold the entire time. Work on spinning smoothly with no wobbling of the knees (try not to have sudden on/off power transitions). Lemond has a good analogy in his book, imagine scraping mud off the bottom of your feet at the bottom of the pedal-stroke (you want to be pulling back, not pushing down at the bottom of the pedal-stroke). Then imagine trying to knee someone in the gut on the upstroke, try to put our knee through the handlebars. Combined with the natural downstroke and the force should be more even throughout the pedal-stroke.
week 5:tempo Medium-distance rides, 1-2 hours 20-35 miles. Find 5-mile stretch and go at 10% below LT up to LT for 15-20 minutes. Rest a bit and repeat. This builds up your aerobic system. Could even do it on a long hillclimb 2-3 miles. Coast down to recover and repeat the climb. Do this 2-days out of the week, two rest days and one endurance day of 3-4 horus.
week 6: intervals medium-distance rides, 1-2 hours. Intervals by definition are anaerobic efforts ABOVE LT, so you'll feel lactic acid burn and your HR will steadily increase to max-HR. Intervals can be 1-5 minutes in length. Pace yourself at a steady pace that'll have you be completely spent and blown-up 100% max-HR by the end. A 1-minute interval can be 98-99% of an all-out sprint. A 5-minute interval will be painful at 90-95% effort. In all cases, you'll want to be completely spent by the end. If you have anything left, go a little faster next time. If you blow up before the end, slow down a bit. Recover fully and do another one. Intervals can be set up any number of different ways, begin with a two 2-minute ones perhaps (2-2) or you can do a pyramid 1-2-1 and later do a 1-2-5-2-1, etc.
Hill-intervals are also fun. Pick a 0.25-1.0 mile hill and pace yourself to blow up at the top. if you've got anything left, go faster, if you blow up before the top, go slower. Coast down the other side to recover, turn around and do it again, and again...
week 7: rest Lowest mileage and effort out of all the weeks. If you're fit, you can still do one small day of intervals, and a sprint-day and an medium-endurance day. If you're not fit, take the whole week easy.
Then create another macrocycle like above, but modify the distances & intensity depending upon where you are in fitness and where you want to improve.
