Hey @dandohan , that makes sense! 

I think you can achieve what you're looking for by utilizing the features of the paid subscription. 

https://support.strava.com/hc/en-us/articles/216917657-Strava-Subscription-Features

Specifically:

Your segment results (this allows you to better compete with yourself rather than others)

Filtered Leaderboards (in case you do wish to compete, you can do it by age, weight, etc,)

Training Log - It's visualized to show your progress, therefore you can see if you're maintaining.

Custom Goals

Training Plans: For running and cycling  - most people use these to try to improve, but there's no reason you couldn't set the goal on par to keep maintenance. 

Relative Effort 

Fitness & Freshness: Track levels of fitness, freshness, and fatigue over time. Again, could easily be used to maintain a baseline. 

Thanks Eric. I'm on a paid plan and appreciate the suggestions. Definitely have found relative effort & fitness/freshness helpful to look back at where I've been.

I couldn't figure out how to customize the training plans to chill them into a focus on maintaining. The ones I see (climb Galibier!) aim at improvement. Can I tweak those or craft my own plan? Am I overlooking some button/ option?

There's definitely room for improvement in the training plans area. For example, a powerful and FULLY customizable training plan platform is https://www.trainingpeaks.com/  It would be cool to see Strava adopt similar features. Training Peaks can be used at the same time as strava, with data uploaded to both simultaneously. 

But with TP or any other program, most athletes are looking to improve, so that's more of the focus. 

So I will get a little philosophical (you probably know this stuff, but sharing for everyone's benefit). As you get older, eventually you have to train harder in order to maintain the same baseline. So even if you're not looking to compete, don't underestimate the importance of striving for improvement. 

Using the existing tools in strava, I'm thinking you could set the bar reasonably low to achieve your maintenance goal. 

For example, look at cycling plans:

https://www.strava.com/training-plans/cycling

You have 6, 10, 15, 30, 45, 60 min climbs. Which level can you ALREADY hold your threshold? 

Let's say you can already keep threshold constant for a 10 minute climb. According to a typical training plan, that means you would want to work on the 15 minute level next. But instead, you could simply keep doing the 10 minute version. I would think by achieving those workouts, you could ensure that a year from now your 10 minute hill climb is still 10 minutes.

If you never increase the intensity (more power) or load (moving up to the 15 minute climb), you likely won't improve, therefore meeting your goal of maintaining. As you get older, you'll have to work harder to keep the same intensity and load. 

You did mention not wanting to go faster, due to potential injury. As you get older, you'll have to ramp up your relative intensity to maintain the same speed. So you would need to be ok with that, or be ok with getting slower over time. Another option to consider is other types of fitness goals. Some longer rides, maybe some cross training, etc.

I think setting weekly mileage goals would also be more helpful than annual goals in terms of maintaining fitness.