Thanks for your question, Donnie. The intensity of your exercise should be such that it elicits an adaptive response. That is, cardio should be such that fitness level in terms of maximal oxygen consumption goes up. Walking alone won't probably do that. So intersperse your walks with more intense bouts of jogging or at least very fast walking. What I said about adaptation applies to strength training, too. My routine (I'm 67 too) is 5 times per week 45 minutes cardio (of which three times are HIIT as described in the post) and 45 minutes strength training.