If you legally are in Canada (visitor's visa, etc) and marry a citizen, the marriage is legal, provided you are not-married or legally divorced (whichever applies to you).
Once you get your marriage certificate, then you can apply for the PR via the outside or inland route. There are advantages to each, so look at the links provided above to help decide which is best for you. My wife and I chose the outside route.
Lastly, there are no stats to give you as to your chances, either in general or in consideration of a gap in age from your spouse. What makes or breaks your application is the evidence you can assemble and send in to "prove" to the government that your relationship is a genuine one.
