Is your group 24 marine battery a deep cycle or a starting battery? "Marine" as far a I know just means that the plates are attached both top and bottom and can be Starting, Deep Cycle or Dual Purpose. It seems if it was a deep cycle you should be able to get more than 3-4 hours of slow trolling, especially in a canoe. If it's a starting only it would probably only last a couple of years as a trolling motor battery but if it's a deep cycle it should last for 4-5 years, I would think. Of course this depends on if the battery gets fully recharged after each use and the water level is kept up in the cells.
A group 27 Deep Cycle only battery should allow you to troll all day and even have some left for the second day if you are camping. I've had luck with the $100.00 Wal Mart batteries and really like the 24 month straight exchange if it fails waranty.