Mr St John, who celebrated his 76th birthday on Saturday, recently had both his bladder and prostate removed. He has suffered another setback after being told he needs more surgery to treat the disease.
"The prognosis is not good because I've got to go back," He said. "But if they can sort it this time, then hopefully..."
Motherwell FC, where he began his footballing career and went on to score 80 goals in 113 appearances, offered words of encouragement.
"We were saddened to hear of Ian's news but also heartened to read of his determination to overcome it," said Motherwell general manager Alan Burrows.
"We are sure he is in the best possible hands and everyone at Motherwell Football Club, and the Lanarkshire community at large, would like to wish Ian all the very best."
Mr St John, who also netted nine goals in 21 appearances for Scotland, was suffering from bladder cancer for several years before it spread to other parts of his body.
"I've had bladder cancer for the last two or three years but it spread into my prostate and therefore the decision was made to remove both of them. That was done 8-10 weeks ago."
Mr St John, who played for Liverpool and now lives on Merseyside with wife Betsy, 67, said he was determined to fight on and revealed that he has not given up hope. He said: "There is nothing you can do about it other than battle on. There is no point in crying about it and saying 'poor me', you just battle on.
"People who have had it will tell you, 'you can get fit again'. I'm getting back to a certain fitness level and gradually getting fitter by the day."
After retiring from football in 1973, Mr St John went on to entertain millions as part of a TV double act with former England striker Jimmy Greaves. Their Saturday lunchtime football show, Saint and Greavsie, was famous for the catchphrase 'it's a funny old game'.