20 to 30 minutes a few timew a week isn't much stretching for a martial artist, especially if you're also lifting. I think that could definitely make a difference in how fast you recover.
Try stretching when you're watching the news at night or doing homework or reading or whatever. Instead of sitting on the couch, put your back to the couch and just hang out in splits for a long period of time. Switch around here and there to other stretches and just completely relax into them so it doesn't even feel like you're doing them. Nice and slow, not forcing it. Do it like I used to practice sleight of hand coin tricks when I did magic -- while I was doing other things.
I'd also recommend doing pistols, the one-legged squat talked about in the Naked Warrior book. There's a program there that is called greasing the groove, where the idea is that you do the exercise frequently, but to notably less than failure. Just do five of them, or less if that's all you can do, and if you feel you have more energy, just do more sets, don't crank out more reps. "Strength is a skill" is the mantra, and you actually do have to use balance and body awareness to do a one- legged squat, so it fits pretty darn well into martial arts, actually. I'm an old time martial artist too, and I've been using the pistols to get back in shape again. They really are good strengtheners that you can do anywhere, anytime. And there's a kind of fun feeling of accomplishment in doing them, as there's some show-off value there to go along with the solid strength building.
The exercise is open-ended, too, because you can add weight to make them harder. Since balance is involved and you're using a lot of stabilizer muscles, and you're only using the one leg, every little bit of added weight makes it much more difficult. There's a guy named Steve Cotter who does them with two 88 lb. kettlebells at once. Pretty fiendish for a one-legged strength/balance/flexibility exercise. I'll be buying his kettlebell DVD's myself; he's a kettlebell trainer and full contact martial artist himself. His site is Fullkontact.com. (Yeah, with a K)