Skip an endurance day....

...and add in another strength day. Or, if you don't do any strength training, use that day to start now. Strength training is so important for any age endurance athletes but especially those in he masters divisions. Adding strength training to your program only during the winter months is pretty useless as the benefits will be gone in a few weeks once you stop again in the spring.  Strength training is a great way to prevent over-use injuries, correct imbalances, and improve the efficiency of your effort. The old 'I don't have time' excuse doesn't work. Skip a run, ride or swim and add in some time doing lunges, squats, pushups, dips, pull ups, planks, etc. The benefits will greatly exceed what you would have gained from that day of endurance training.