Just go. The more times you go and come the back alive the easier it will get for her to believe you can do it safely. She will not stop worrying, but might stop harassing you about it. If it is easier let her think you are going to the middle school and just doing lots of laps. It sounds like it is in the same area as you want to ride anyway. The main reason I say this is if something did happen you want people to start looking for you in the right place. Generally if I am going more than 15 miles round trip I let someone know where I'm headed. Otherwise I just tell them I am going riding and will be back around X:XX time. One thing you may wish to bring is a cell phone. If you have one. She might feel better knowing you can call 911 if you need to. Also you can tell her I have ridden over twice that far many times and am still here with all limbs, features, and faculties intact. I do wear bright clothes and a helmet. I also always expect to not be seen. That way I'm ready when it is true.