Running with a loaded buggy :)

Hello runners I usually run 10k in the evenings 3x per week. Today I fancied a day time run so I loaded my twins into their double-jogger and managed a flat 5k. It was really, really tough. My shoulders suffered the worst - is this potentially damaging? TBH - my main question is this; does anyone know the calorific benefit from doing this? The kids weigh 11kg each and the pram is 18kg so I was pushing a 40kg load - if it's worthwhile I'll happily do it again as they loved it, but if there's no benefit or if its negligible.... well, no thanks!! Thanks for listening - any thoughts welcomed