In reply to Double Knee Bar: I'm not overly convinced that ankle weights will do much other than alter your gait and leg muscles - I don't notice much difference in cv work load when using heavier walking boots compared to fell shoes (when walking rather than running) but just how tired my calves feel at the end.
I'm not sure about having a slightly heavier rucsac - I do a bit of running with one (e.g. running from work) but largely notice it slows me down a little rather than makes me work harder, but as they're normally recovery or otherwise easy paced runs I'm trying to keep the effort fairly low.