If he were polling far ahead of Hillary Clinton in 2000, he never would have dropped out of the 2000 Senate race.
Yes, he had cancer. But he had a very common form of cancer and was undergoing a treatment regimen that has almost impact on the patient's day-to-day routines (witness John Kerry in the 2004 presidential campaign).
I know several people who have had prostate cancer. Their treatment included surgery, chemo, and radiation. All of these forms of treatment were debilitating and impacted their functioning capacity for a time.