I'm not naturally flexible and so I would agree with what's mentioned previously, stretch tons. I stretch any time I'm in front of the tv and before I go to bed. And rest is not overrated!
One other thing that hasn't been mentioned yet: I'm sure you've been fitted, but something to consider is how your fit may have evolved due to your higher mileage these days. For me, moving my saddle back helped me get down lower on the bike, but it also took some of the emphasis off my quads and shifted it to my glutes and hamstrings. If your quads are truly dead all of the time (granted this is very subjective) you might have people check out your fit, or experiment yourself with fore-aft.
Good pedal technique also goes a long way I think. Regardless of where you are on the saddle, you can decide to emphasize different leg muscles during the pedalstroke.
I don't think you can ever eliminate quad soreness, but there are ways to mitigate it during rides, and strengthen the surrounding muscles as well. ymmv!