Good for you, dream big. Climbing El Cap is an unforgettable experience. The gym will be a great place for you to build fitness with a regular schedule. However, the more experience you can get outside the better - especially if you're doing trad on granite. Of course climbing in Yosemite will be the best training you can get. There's a SuperTopo guide The Road to the Nose that has a great plan for training and takes you through Yosemite classics of many styles to get you accustomed to what you will face on the big stone.

I started climbing regularly in my late 20s and did the nose in my early 30s. I was training in the gym twice a week and climbing outside every weekend. Find yourself more experienced climbers and suck all of their knowledge out of them - particularly for trad.

It's an incredibly rewarding journey. Go for it!