Short answer seems to be, "accept it, and fight it."
Funny story, I've likely told before. I've posted about my friend and teacher, Anthony Davis. Anthony was 86 when we met, in fact, it was on his birthday. I'd just turned 36 myself....I asked him, right out, What's it feel like to have gotten that old?....guy looked like he was gonna hit me for a second-Anthony was fierce!....instead, he replied with what I've come to call "Anthony's Curse."
I feel just like I'm 16, and there's something really wrong with me.....
It's Anthony's "curse," because every couple of years (each decade, now, even) since then, I know more and more what he meant.....First thing I'd say you really have to start doing is measuring and metering everything-while your endurance won't go down much, it's best to hold back a little on some things to preserve them, and prevent injury: you probably don't take many falls in TKD, but, for myself, one way I've done this is actually limiting the number of falls I take for my jujutsu students-I keep count, and stick to that number.....pretty soon, now-likely some time after 60-I reckon I won't be taking more than one or two....in any case, figure out where you can concentrate, and where to cut your losses-with you, it's kicks: is that 720 degree jumping spinning kick really worth even trying for at this point? It's hard on the hips, and not really something that you're going to "need." If you can already do it, then you're probably not going to get much better at it, at 48-save it for demos, and practice it when you have one coming up-but as little as possible. If you can't do it yet, well then, forget about it. That's right-you're 48 years old, and if you haven't mastered something requiring such a level of athleticism, you're not gonna....not kicking fast and high, well-how can you kick and remain in the game? What's "fast?" What's "high?" Could you be a little faster if you aim lower? Just sayin', 'cause-and here's the sad truth: you will not ever kick faster and higher than you do now.....
It should probably be likewise with your sparring-start limiting those sessions, because it's just gonna take you longer to recover. Tony's article was right on all counts-yoga-just stretching for stretching's sake-will not only help you maintain flexibility, but help prevent injury and maintain muscle tone and range of motion-if you do have that helicopter kick, this is more key than actually doing the kick.....
I guess, though, for me-and Tony's advice about "going with the flow" follows this-at our ages, a "less is more" approach is called for-it's kind of up to you to figure out for yourself (by listening to your body) what that means....but in free practice, it typically means altering strategies and tactics to accommodate changes, and focusing on what we're good at. I've had physical issues-three deep-vein thrombosis, that's blood clots, in the same leg-that have made me have to abandon some of my old standbys: I shouldn't-and don't-deliver shin round kicks with that leg anymore...which is too bad, since that shin to the thigh could take a lot of fight out of people, once......I'm working through that, though-turns out (surprise) my left leg isn't discernibly weaker than the right, and not much slower.....leave off those things that are more likely to lead to injury, or that simply aren't going to be "go to" items for you, and concentrate on the go to stuff-this is, after all, the essence of mastery-to perform those things that you have perfected in a dominating fashion....
A lot of martial artists in our age group-especially those in karate or TKD-wind up with leg joint problems-hip and knee replacements aren't uncommon. Same for judo...I count myself as lucky that I don't see any of those in my (immediate) future, given the use I've had of them, over the years. A couple of more trips to the doctor are called for, at our age...getting ahead of issues like this is key to maintaining any kind of activity.
Lastly, I'd say start looking at supplements-our bodies stop producing or processing stuff we need at this point (actually, before) in our lives, and there's a variety of legal, over the counter substances that can help some with this-though there's a lot of argument about some of this, so I'll leave the hunting down of what works for you up to you.....
To minimize soreness, take an anti inflamatory - Ibuprofen with a little food, half hour before workouts. Ultra marathoners in great shape use the pre medication technique. If experiencing Joint sorenss after workouts use ice. Walgreens sells a velcor wrap with a pocjket that holds a re freezeable pouch. I would wear that for 20 minutes after a run. 
