I turn 60 in a few days and I've been riding since my late 40's. I've gotten in pretty good riding shape in those years, and have extended that to running over the last few and I'm fairly strong at that now.
I'm sure that Friel's books are insightful, but I'd caution that virtually everything on the internet, or conventional training dogma, are geared for the 20-40 age range and at best inapplicable for our age. In my opinion, our primary objective above all else is: incremental improvements. NO formal training schedule beyond distance and pace goals, no scheduled high intensity intervals. We have to be mentally flexible enough to change the plan on a daily basis, based on evaluating our instant state of recovery while disciplined enough to achieve enough training stress for adaptations. The two lines get closer together the older we get, and no planned-out training schedule can accommodate that for older athletes not already in peak form.
We have to ratchet it up. Stress, adaptation, healing of whatever takes the biggest hit, and THEN ticking up the performance goals, normalize that and repeat. It's a totally different pattern, a different paradigm than for younger athletes who can afford to take things to limits and then power onward a few days later.
Because of that I'm not a big fan of HIIT. I say, sprint when you feel like it. Vary the distances and intensities. Hit it on hills. But do not start out with a goal of X number of 1 minute all out, 1 minute rest nor any other pattern like that. The recovery period will drain the gains. But do the sprints when we can, 1 minute, 20 minutes, whatever, we will gain from that also without crossing a line that stops us for a day or days.