Seeing that changing your physiology and adapting to training stress is paramount to being physically prepared for race day, if your training plan asks you to complete x-miles and there's minimal structure or purpose to the workout, you are delaying the opportunity to experience significant gains in fitness. Additionally, your training will become monotonous and you'll find yourself going through the motions, week after week with little to no improvements.
Furthermore, if you aren't comfortable riding your bike due to a poor bike fit, you do not know how to use sport nutrition properly to stay well fueled and hydrated and/or you lack the proper skills (climbing, descending or changing gears) to ride efficiently on your bike, you will struggle to improve your fitness (and you may risk injury or sickness).
While there's nothing wrong with either of the above statements, athletes should not make the only goal of a workout to see how fast you can go or how far you can go.
Instead, focus on what's happening within those miles and above all, be sure that you can actually absorb the training stress that you are putting on your body. With this comes a responsibility that you are incorporating great lifestyle habits like good daily nutrition, good fueling before/after workouts, great sleep and stress management and the ability to function well in life.