I do two types of intervals. The interval is 8-10 minutes followed by 5 min recovery. The first one is heavy resistance with a slow cadence (under 60 rpm). This is called "muscle tension" and it really builds up strength in your legs. The other is a tempo interval -- find the resistance/cadence that takes your heart rate to a place just a touch below your threshold and maintain it for 10 mins. On the tempo interval, gradually work up to 20 mins. This is building up your cardio training and learning to how to ride longer without burning up your legs.
There's other lactic threshold intervals, etc. that are also helpful. You will find that year over year you will get better and better. I am 58 years old and ride 5 mph faster today than I did 2 years ago. So, I would imagine a younger person could gain fitness and improvement fairly quickly.
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