I can't endorse this post highly enough.
Arse Cancer, in one form or another, has done for many of both sides of my family. I had a problem a few years ago (an experience I wouldn't wish upon my worst enemy) and now have a colonoscopy every three to five years or so.
(The procedure is painless but the day of fasting and drinking laxative that tastes like sea water is goddam fucking awful).
The other very important thing I would add is to include lots of fibre and oily fish in your diet. This needn't be a sacrifice or even an overly dramatic change - choose brown bread instead of toast, eat chowder instead of soup, have kippers or mackerel instead of a fry up etcetera.
This was the advice that the doctor gave me when he found polyps six years ago. I followed it and, thankfully, there were none since.
The good news is most of these conditions are eminently treatable when they're caught early. So, for fucks sakes, man up and get yourselves checked.