You should be able to see consistent improvement for a long time – it’s great that you have no immediate goals but are looking to build fitness and capacity. Taking a structured plan like that 24-week mountaineering plan and repeating the base period would be a great idea.
Consistency, gradualness, and modulation are key: https://uphillathlete.com/dreams-are-not-goals/
Spend as much time as you can well below AeT – targeting 10-15 beats below will be a good conservative approach. Slowly improve your strength. I’m not breaking any records, but was able to make really substantial progress with a schedule that looked like this:
- Monday: 30-40 minute Zone 1 jog. 30-40 minute core strength workout (Scott’s Killer Core, easy version), maybe some lunges, etc.
- Tuesday: 30-40 minute Zone 1, maybe a little bit in Zone 2
- Wednesday: 30-40 minute Zone 1, maybe a few striders (10-15 second accelerations)
- Thursday: 30-40 minute Zone 1. 30-40 minute core strength workout
- Friday: 30-40 minute Zone 1
- Saturday: Long run, 60-90 minutes Zone 1
- Sunday: Recovery – rest, walk, or 30-40 minute Zone 1
Really basic, really conservative, and real improvements. Here’s a writeup I did last year on some of this: https://www.reedshea.com/posts/2020/2019-training/