I don't understand why everyone in this thread so far is against a weight vest on a bike. I've done something similar with a backpack full of random weights (which I don't recommend for more than 5-10 miles at a time, I had to do that because I'm just out of college and very poor) and it's a killer workout. Weights vests are great because they add intensity to almost any movement (especially vertical stuff) and they distribute the weight much better than my ghetto setup. It's reasonable to be worried about your back, though, so start at 10-15 lbs or so to be safe and then add weight from there gradually. But really, they're great even to use on a walk and I fully plan on purchasing a proper one when I have the money.