Writing is an art very few perfect. Because it not only takes time, but months on end sometimes even years to complete a thought that is streaming through your mind. The best of writers in centuries gone by like William Shakespeare, Jane Austen, Charles Dickens, Bernard Shaw and many more have had to sweat by their brow for years on end until they perfected the art and many more years until they received recognition.
The art of writing stories is to know the basics. Perfect your foundation. Learn the definition of the plot, structure, conflict, perspective and climax. It is only when you have written and rewritten all of them and studied an author you admire, only then will you be able to learn the rules.
Once you have learnt the rules, get ready to break them. It is only when you have mastered the art that you will be able to complete a book with all its richness and variety.
The characters in your story are the flagships of your idea. However you will be able to round a maximum of two characters. So think whom your readers need to know thoroughly before you endeavour further. Also a telltale of more than 25 years of a character pressed in 200 or 300 pages sounds unrealistic. So make sure at the onset what about the life of the character you would want to interest your readers and accordingly get writing.
Do not expect your first book to be the best seller. Once you are done with the story, rewrite it and check if the second written version is better. After that rewrite again. Then begin your hunt for a publisher of your taste. Sometimes the very first knock works while sometimes it takes months.
So do not give up on yourself ever. Believe in yourself and believe that there is nothing impossible. The road may seem winding but the journey is always rewarding.