i didn't know you were in this movie vike

I believe there's little evidence that says people can significantly change their muscle fiber composition (fast vs slow twitch) through working out. I honestly haven't kept up with the latest research on this, but if anyone has facts stating otherwise, I'd be intrigued! What generally happens from specific exercise is a combination of muscle hypertrophy (greater force output), fat loss (better power::weight ratio), mitochondrial growth (more efficient energy consumption), a variety of adaptations in cardiopulmonary and endocrine systems, improved coordination/muscle memory/proprioception (more efficient movement thru more efficient, precise muscle contraction). All these things occur in tandem to result in overall performance improvement. The physiological science is generally well understood, but the application is the real trick and where most people have trouble.
Everybody's correct about hard work, good discipline/adherence to a program, and functional training (as in no substitute for the activity but the activity itself). Essentially everything besides the activity can be considered cross training, which is beneficial for emphasizing individual factors of exercise adaptation (as written earlier) that may be slower to develop if performing the activity alone. The body is amazingly capable of adapting to repetitive activity (or lack of activity for that matter).
General fitness comes in all sorts of flavors and crossfit is perhaps the newest craze. "Fit" is a relative term. Sometimes athletes need to be heavier in order to create enough power in speed/contact sports. In motorsports, the machine does the acceleration and your finesse through inputs is key. Being able to deadlift 500lbs is not what I'd say to be a good measure of "riding fitness" unless you need to quickly haul your bike out of the hay, get back on the track quickly to finish the race .
Ultimately the best program is the one you enjoy, is directly applicable to your goals, and is one you stick to long term without injury