Jen-
I’m also well north of 50, at 57. I too had to take a decade+ long break in my late 30s due to a medical condition, only to return to tri at 47. I get it. That said, It’s still possible to gain run fitness and some speed. I don’t know what your ultimate potential was or is… But, I’m sure you can improve.
But, what you are experiencing is just poor run fitness; but good general fitness, mostly from your bike. That’s why your hr remains lowish—you are fit, just not run-fit. Running is unforgiving. With only one-ish run of 5-8 miles per week… It’s going to be very hard to make much progress.
Changing your run gate (raising your knees, etc) won’t make you faster. Trying to do speed work other than doing basic strides, more than likely it will only make you injured—especially, as someone with your ankle and arthritis. That’s putting the cart before the horse. High knees, and a strong heel kick (aka pretty running form) are a consequence of fast running not the other way round.
What would serve you best is just another day or two of running per week, and maybe some strides if you aren’t doing those already. But, I don’t know if that’s a good idea for your arthritis and ankle reconstruction or not. For that you need a medical opinion…as for what’s best for you long term.
Honestly, I wouldn’t do bricks, unless it’s a time management thing. There’s not any fitness benefit to doing bricks, and from a run point of view, it puts you running on tired legs, and will only cause you to run slower. I’d do solo runs as much as possible. Running on fresh legs will make it easier to run a little faster, and will benefit your run fitness more - - - assuming the same workouts are completed…just separately.
Obviously, I don’t know anything about your arthritis or the nature of your reconstruction. But, I do have have running friends (of a similar age) with arthritis and other reconstructions, who have managed to return to regular, frequent running with proper treatment. Again, a conversation with your medical professionals—as this is all very individual. None of us can(or should) advise you there.
Running fitness… Even with enough frequency… Takes a long time to regain.