Paddling can be a very strenuous sport. As we get older, we often spend many more hours at work and less time on exercise. You can compensate to some extent with experience - experienced paddlers learn to work with the river instead of trying to overpower it. When was the last time you practiced aggresive swimming techniques? If it has been awhile, give it a whirl on a local trip. Do you practice rolling? Eric Jackson recommends practicing your roll on every river outing. Taking this course is a strong commitment towards building your river skills. There are many other sources available as well. Practicing tough lines on easier rivers is a safe way to develop solid boating skills. Tom McKewan is a strong advocate of river attainment exercises - a great way to learn how to read rapids and build stamina. When was the last time you performed a gear check? Are you absolutely certain your PFD will still float you adequately?