I dont excersize much and now my legs feel funny?

I do yoga once in a blue moon. A few months ago I started a excersize regimen by watching FIT Tv and doing the excersizes, and then I got lazy and didnt do it anymore. It's hard to tell myself do the excersizing and I get discouraged when my body cannot do much until it gets tired. Recently I notice my legs get weak and my mum says I need to excersize quick before I develop something bad. What can I do to get my legs feeling stronger? Is it common to get weak muscles if you dont excersize besides walking and stretching?