When I was a nerdy girl who didn't focus much on my appearance, I used to think that there was some advantage to dressing “practically” over “prettily”.
A girl who didn't fuss with her appearance was ready and willing to go on any kind of adventure on a moment's notice. She was the kind of girl who would the boys would want to play with, because she didn't care if her hair or makeup got messed up. Of course, I was only trying internally justify my unwillingness to improve myself.
After a re-haul of my wardrobe and look, I started wearing make-up, heels and skirts. I started wearing these kinds of clothes almost everywhere I went. I wore these clothes to work, to the grocery store, and to the doctor's office. However, I always kept a pair of sneakers in my closet in case the occasion called for them. So, at a moment's notice I could go hiking, or camping.
There is a misconception that in order to become feminine, we have to be only feminine, and give up the parts of ourselves that may not be traditionally feminine. However, in spite of wanting to be feminine, there are times when our other traits can come in handy. Our willingness to be able to go outside and play with the boys does not have to go away when you learn how to behave more femininely. In fact, men love a feminine woman who also has the ability to relate to them. It's like having an extra special-something that makes you unique. Like frosting on a cake.
Yes, be pretty. Yes, be feminine. But also be willing to share in the activities you can do with your SO. I go hiking, go-kart racing, and indoor shooting with my SO. He thinks it's neat that I am a decent shot, and that my degree in STEM lets me understand when he speaks math to me. These things aren't necessary for attraction, these are the nice-to-haves. It is not only okay to be able to do these things, it is good.
It is good to have a pair of sneakers in your closet. They don't have to be your everyday shoe choice, just store them in your closet in case you need them.