Problems with leading ponies?

I sometimes help out at my local stables and i am asked to lead ponies in the lead rein lessons. but when i ask the pony to trot, i find that it will just speed off (any pony) and i get left behind or the pony will pull to go faster. i have tried running faster and pulling back harder but these just in courage the pony to trot faster. i don't really feel in control. do you have any tips for leading a pony? thanks