No, he didn't hold off the regeneration itself for 100 years - that was only between getting irradiated while saving Wilf and leaving Rose (so however long it took him to go and visit all his ex companions). The century comes between The Waters of Mars and The End of Time.
During Waters of Mars, he knew it was coming, and his Tennant body was almost done for. 100 years is quite a bit of time for it to be almost time for him to Regenerate, so, I gotta think the "Holding it off" started way before End of Time Radiation, that just added fuel to the fire.
No, no, no. The radiation was killing all his cells. That he held it off for however long (a day, a month, a year, 100 years) to visit everyone was what caused the catastrophic regeneration. The 100 years that is mentioned above is between when the Ood called for him and when he showed up. 100 years for the Doctor, not for the Ood. Coincidentally, he arrived in Oodville 100 years after he release Ood-Brain.