abv8:
First, you have made a wise choice in choosing cycling. It's the best sport there is!!!
Let's take a look at what you have to be good at in order to succeed in road cycling.
As a road rider, you have criteriums, road races, and time trials. Crits are basically riding in a circle, usually under a mile for many laps, road races can be over varying terrain for varying distances, usually 50-100+ miles, and time trials are races against the clock, usually 10-25 miles.
You need lots of aerobic conditioning for all three diciplines. Crits also require lots of anaerobic fitness to handle the necessary jumps/accelerations and finishing sprints.
Road races will also reqire anaerobic fitness, but you have to be able to climb as well. A hilly road race is usually disasterous for a non- climbers placing and confidence. Long road races also require extra endurance compared with a crit.
Time trials are usually over flat terrain, require lots of aerobic power, the ability to withstand constant lactate presence, and lots of leg, hip and buttock strength to turn bigger gears for extended periods of time, usually 20-60 minutes.
Of the three diciplines, time trials are known as the "race of truth" since whatever you accomplish time wise for your distance tells almost a complete story of your fitness level.
A 10 mile TT at 30 mph is around 20 minutes. That's about as fit as most humans could ever get if everything with their training and genetic talent were ideal. A 10 mile TT at 30 minutes is a very low level of fitness, around 20 mph. That's why TT's are called the race of truth. You and your bike are on trial and the stopwatch is the judge of your current abilities.
Many crit riders can hold 30 mph in a pack of 60 riders, but probably none of them could TT at 30 mph for 10 miles by themself (unless they are TT specialists). However, good crit riders usually have awesome finishing sprints, that leave most time trialers finishing poorly somewhere in the pack.
The bottom line is riders need to find out what diciplines they are best suited to and basically stick with that. Find out what you are good at and go from there.
Lower cadences stress the leg muscles more, higher cadence stresses the heart more. Generally, for road riding, work between 80-100 rpm for the most of the time. During the work week, stick with 60-90 minute rides indoors or out, and do longer rides (2-4+ hours)on your days off. One long ride per week is good.
Before starting intervals, just ride more, and gradually increase your average speed on rides. Go on group rides, and join a club if you want to race. If you get the chance, ride with faster/better riders whenever you can and learn from them.
At 20 years old, you still have time to become a professional and have a good career if that is your goal.
Good luck!!!