Agree completely with Xcel.
Light weights will not allow you to retain muscle as well as heavier weights. Running tends to be a catabolic stimulus for most BBer's. Dorian used to use brisk walking for aerobics, and my personal experience has been that this works well.
Go to the Articles Forum, and look at the "Dan DuCheine Post Recovery" thread. In that is a description of DuCheine's Ultimate Diet, which has worked VERY well for retaining muscle while eliminating fat.