The improvements you are looking for come mostly from the bike. At your age, I found the greatest improvements from doing one 60 mile, 3000' (or thereabouts) a week. I'd go moderate on the flats and as hard as I could on the climbs, trying to get so tired I could hardly walk by the ride's end. Leave it all on the road. I'd get about 45' of Z4, 5-10' of Z5, 2' Z1, some Z2 and a good bit of Z3. That would be all my intensity for the week. The rest of the week was 2 days Z2 and 2 days gym, different gym workouts on the 2 days, about an hour each, last set of each exercise done to exhaustion, not failure, just didn't think I could do another rep. That was only ~100 miles/week, usually 8-12 hours/week depending on season, some weeks even easier. The gym work was to increase endurance which it did, not so much speed on the bike. That comes from the bike.

The program allowed me to turn in good times (for my age) on mountain rides up to 400k, without doing any longer rides. Obviously I was quite fit to be able to do this, simply because I'd been doing that since my late 50s, year-round. I once took a winter offf and deeply regretted it. I took me the whole summer to get it back.

You can start by scaling that whole thing back to where you're able to complete all-out hilly rides of whatever length. Then you increase that length by 5%/week. Some books say 10% but that's a crazy rate of compound interest. Do the gym and Z2, whatever volume seems to be sustainable for long periods. I'd take an easier week every 2-3 weeks but would do my normal effort in the gym.