It's a woman. She's not going to be interested in your "bro science" or "hard work". She wants to feel good about herself and be showered with attention, without breaking a sweat.If you walked 3 miles you probably burned a little more than 300 calories. This will certainly help, but what will help much more is taking an accurate inventory of the caloric content of the foods you routinely eat. If it's not something you think about there may be hidden calories in your diet it would be really easy to cut out or substitute for something else. Black coffee instead of 200 calories for a sugary coffee drink for example, or being aware that an Oreo has 50 calories per cookie so if you are going to eat an entire sleeve be aware that that's 600 calories of almost pure sugar and fat when considering other food options that day.
Walking also takes a long time. You would benefit more and quicker working out a strength training routine. It takes less time out of your day, you can get similar cardiovascular benefits from elevated heart rate activity, muscle is more metabolicly active requiring more resting calories to maintain, and the activity of your body building muscle uses carbs. You don't need to worry about extra muscle making you look 'bulked up' for quite a while if you are just starting out, but if you get to that point just stop lifting progressively heavier weights. Once you have the muscle it takes much less exercise to maintain it.
Walking is great, but changing your body composition through strength training will be much more beneficial than trying to slim down with light aerobic activity.