endurance running 1 time a week is not enough to build the actual endurance. like speed training, endurance for running diminishes very quick. waiting 6 to 7 days to do it will not allow for it to build up to any appreciable level. if you want to run for endurance, i would probably set aside 3 days a week for it. just to let you know though, the form, tempo, and muscles used in endurance running will conflict with cns innervation for explosive sprinting meaning by trying to gain in both, you will be negatively affecting the other - mainly your sprinting will get worse.