It is a journey. Think of it like trying to learn a new language. Saying I want to speak fluent French doesn’t have any result on my ability to speak French. However, speaking French every day with people who are more fluent than I am, watching French movies, listening to music sung in French, reading books written in French… that would have a dramatic impact.
Learning to sketch is the same. At my first job I sketched 50-60 hours per week and I still came home and sketched 2 hours a night and on the weekends. I did that for about 4 years. I tried to sketch with the senior designers as much as possible. When someone did a sketch better than me I photocopied it and put it in a binder to keep as reference and study. I showed my work to as many designers as possible and got their critique… do that and you will improve. It won’t be able to not improve!
Now there are so many more resources. YouTube videos, the core77 sketch forum. If you have the discipline and put in the hours you will be good in about 2-3 years of daily work at it.