You know what you have to do. I'm just a year older than you and have been working on keeping up with my betters ever since I started riding again at 50. Here's what works for me:

Weekend rides: I do one, 35-55 miles in the winter on tandem or single, just because the weather is not so great up here and because it's good to have an easy season. In the summer, I ramp it up from 60-100 miles on my single, or 60-80 miles on our tandem. These are all group rides with some riders faster than me or us. On my single, I ride at the front a lot, on the tandem at the rear a lot, same group.

During the week, I ride my rollers on Tuesday, either FastPedal or one-legged pedaling intervals with some Z1 warm up and cool down, all seasons. Wednesday I'll do an hour of Z2 on the rollers, no matter the season, just to save time. Thursday we'll either go to spin class or out on the tandem for 20-30 miles, or out on my single for 50. Friday, a short Z2 on the rollers, 30'-45', or nothing depending on how I feel. Saturday nothing, Sunday the group ride.

On the group ride, I'll do some LT work all year, but ramp it up in the summer with hard efforts on every climb, trying to hit as close to LT as I can, depending on how long the ride is and how I judge my survival ability. I'll also do some hill sprints and anaerobic hill efforts say from May on, gradually increasing as my fitness nears its peak. In the summer, if I can walk at the end, I could have gone harder. I go for total depletion, but try not to bonk, doing my 250 cal/hr. On the tandem, Stoker has a higher LT than mine, but holds my HR so she can stay with me all the way.

I measure my work ability by charting my morning resting HR and HR after getting up and standing for 2 minutes. It's pretty obvious when I can go and when I need more rest. 10 beats up from usual on the standing HR is a take-it-easy or rest signal until it goes back down.

I'm 66", 152 lbs. now, having lost 10 lbs. since December. I lost it by eating smaller portions. I ate much less Monday-Thursday, especially less carbs but held protein steady, then reversed that and ate much more carbs Friday and Saturday, holding the fat down, getting ready for Sunday. I have low blood sugar issues when I get hungry off the bike, which I solved by taking NOW Super Citrimax (garcinia cambogia) when in the low calorie part of the week, but not during the carb loading phase.