what will biking do for me?

I asked my friends this question....but i thought that I might get some more insight. I have been biking with my husband and kids. We each pull one child....and its around 50 pounds for the baby and seat and then the cart. I am wondering if my legs are the only things getting any kind of workout?
I have been doing in for about 4 weeks and we are riding at least 4 miles each time..not slow either....we are going at a very good pace....and we just started this new trail that's all hilly and very hard. (Kids in tow i might add) I have lost almost 5 inches off my legs in these four weeks so I know its helping that.....but is my stomach going to get smaller or my arms from this at all???? I hope this is not to silly of a question! Thanks in advance!