Cheap, Light weight Tow Leash solution that for Running and Kayaking (idea stolen from Ian Adamson's Adventure racing book
Get about 36-40" lengh of 3mm climbing cord and about 8" lengh of the same diameter elastic cord. (the elastice rope reduces the "yanking" effect and makes towing much smoother). Tie together and attach a small carabiner onto each end (non technical, decoration ones for $1 ea. work fine). In most cases the carabiners fits onto the backpack or belt (&; kayak hooks) but in case a runner does not wear eather one, measure the approx. waist size and make a small loop into the leash, so the tow rope can be wrapped around and clipped in without pulling tight. As with everything, make a test run for before the race to get a feel for it and adjust were needed.