When doing intervals, think of time instead of distance. Warm up for at least half an hour, do your interval session, cool down, go home.

The length of an interval ranges from as short as 30 seconds or as long as 20 minutes. The short ones increase VO2max and anaerobic power, the long ones raise your lactate threshold. Some folks advise using interval lengths of 5 to 7 minutes, which is short enough to let you work hard but long enough to accumulate the training time needed to cause improvements. That's probably a good starting range.

Start with just a few intervals and work up gradually. I think most people work up to about 40 minutes of interval training per session. You'll probably want to start with half that much, or less.

Regarding skydive69's "go easy or go hard" training philosophy, that's one way to train. However, there is a training "sweet spot" a few beats below your lactate threshold where one can get almost as good a training adaptation as the 90%+ range, but you can do it for longer. Since you can do it for longer, you can acccumulate more training per day. Some people call this range "tempo" or "fast fun" riding. I've been doing most of my climbing in this range (except on interval days), and my times continue to come down. I also can do multiple "tempo" days in a row without the need for a recovery day.