Whitlock is an incredible specimen. All I can say about him is he’s proof that people shouldn’t set arbitrary limits on them just because someone else says something is impossible. Not that many people can expect to do what he does but the lesson to be learned from him in my mind is what I just stated, don’t set arbitrary limits on yourself.
Fortunately for me, another person much like Whitlock was my coach (or one of my coaches) for the first 6 years of my running life. Until I was out of high school and into the real world, I thought a 50-something year old who could run faster than most of the guys on the high school team he coached was the norm. Unfortunately, that left me learning a lot once I was out of high school.
Walking is a great way to get some supplemental training in. I like going for a good walk in the afternoon after a morning long run. It’s great for recovery, not to mention a little extra stamina training for the legs.