I don't know how old you are, but it's the same no matter how old you are, only it's more if you're older like me. Heat affects you more, extra miles affect you more, and climbing affects you more. The physiology of stressing muscle tissue is out there if you want to research, but for me, it's about my muscles repairing the damage done the day before, flushing the lactic acid and other waste products of muscle activity. Recovery rides such as you described are valuable. Plan on them.