I'd recommending adding in a good 15-20mins per day of weight-oriented resistance training. Can be as simple as using a couple of kettle bells or dumbbells. Pushups, situps, squats, triceps, chest, back. Done right, with a solid amount of weight, done in a sequence of high-intensity bursts (intervals) in addition to whatever cardio-oriented training of other sorts you're doing (cycling, swimming, rowing, running), it can make a big difference.

Helps keep you strong, for avoiding injuries. Helps build lean muscle, for boosting your calorie burn. Helps improve your ability to withstand stronger and longer workouts on the bike, boat, road, in the pool.

Be sure to have enough protein in your normal diet, for ensuring you can effectively build the muscles and recover from workout "damage" you do to your muscles by training.