It's extra stretchy. That is true, but another thing is that running speed is different to movement speed. So when you're attacking you wouldn't be at twice speed, as you'd have to slow down. Besides that there isn't anything that needs to be put out there I believe.
*Tosses you weights*
Wear these and start doing 2012 NB tai exercises for a month.
( Katai Omori ) - Leg Weights
Type: Weapon/Supplementary
Rank: A
Range: Short
Chakra: 30
Damage: N/A (+20 to Kicks, 2x Running Speed)
Description: The Leg Weights used for Special Taijutsu Training with Added Weights are made through the application of a sealing formula to ordinary leg weights. The sealing formula is used to, through the use of chakra, increase the weight of the leg weights to the desired level. The seal can't be applied to anything other than the leg weights and its use is restricted to its original purpose. One can remove the leg weights but the sealing technique in them stays active indefinitely and can't be released. Additionally, the Leg Weights and their unique seal calibration makes them only usable by the ninja to whom they belong. They can't be sealed or stolen.