i guess it would really be up to your conditioning level. there is no fixed rate for everyone because everyone has different range of heart rate. technically speaking, you should not be asking for the specific heart rate, rather, the percentage of maximum heart rate, because every has different maximum heart rate and resting heart rate.
Being long distance paddling, I would presume this is not a sprint event and thus you would do good to stay within the aerobic zone of 70-80% MHR (maximum Heart Rate). Of course the more conditioned and trained you are, the nearer you can push yourself towards the 80% mark without crossing over to the anaerobic zone, within which there will be significant lactic acid build up. You can probably try going faster towards the last stretch, providing you have been reasonably conditioned in anaerobic paddling (ie. flatwater sprint kayaking).
The easiest way to estimate your MHR is using this formula:
MHR=217 - (0.85 × Age)
Of course this is not an end all be all formula, as the relationship between heart rate and age is not exactly linear.
of course other than heart rate, you would want to take note of the strain on your rotary cuffs and knees.
I guess in long distance kayaking, like any other long distance sports events, you have to run your own race, in this instance, paddle your own boat. Preparing yourself by progressing to longer and longer distance within the aerobic zone will be a good guide i think.
