I alternate between emphasizing bicycling, swimming, walking, resistance exercises (and balance training and prescribed stretching). Today I did a training ride (hills) followed by a swim. Tomorrow am I will do an intense resistance session - weights, TRX, etc., with a walk and then a short bicycle ride with the wife. Wednesday will be a longer but easier bike ride and swim, etc., etc. This allows for muscles used by one activity to recover while doing another activity. Then, every now and then I ease off of everything. This is NOT a scientific training program, but one that works for this 73yo guy, and I greatly enjoy it, which, after all, is what really counts.