Using weights at the ends of your limbs is bad. They used to have "heavy hands" where everyone would walk arount the block holding dumbells. The problem with this is that it extends all the joints and loosens your ligaments. This will happen at the wrist and knee mostly (depending on where the weight is). There is also effects on the hips and shoudlers. Once you lengthen ligaments they do not go back. People were complaining about wrist/knee problems like crazy and they stopped recommending this for people. Ask your parent, I'm sure they remember having a lot of people walk around with dumbells, it was quite popular.
I would either use something like a heavy sandbag if I was doing fast sprints or just get a weighted vest if I was going to do something longer. They make great weighted vests that have little inserts for 1/2 or 1lb increments. Having a backpack with weights would concern me just because it will bounce around a lot and may mess up your gait. Also, the weight is only on one side of you and will probably mess up your posture. Save up and get the dough for a weighted vest.
Hell, go to you local TJMAX outlet sore and get yourself a regular vest. Then go to a gun supply store and get lead shot. Rip and fill with lead shot and make your own weighted vest for cheaper.