Slow down slightly ... maybe to 13 mph ... and get out there and just ride 20 miles this coming Saturday. That's how you get through the wall. You just do it. Yeah, you might have to stop for a few minutes halfway through. That's OK.
A question for you ... are you drinking water regularly while you ride? Aim for one 750 ml bottle every 1 to 1.5 hours.
Also, 12 miles shouldn't be long enough to need extra food, but on your 20 mile ride this Saturday, bring a decent granola bar. When you stop for a little break at approx. 10 miles, eat your granola bar and have a good drink of water.
Then on Sunday, get back on your bicycle again ... sore or not. This ride will be short and at an easy pace ... maybe 6 miles at 10 mph. A recovery ride.
Take Monday off, but get back on the bicycle again on Tuesday for one of your 12 mile rides.
The sore you're experiencing is likely DOMS. The best way to work through DOMS is to exercise. If you really don't feel up for a ride, get out for a walk or a swim.
Oh, one more thing ... is your bicycle set up correctly for you? There's a chance all the soreness etc. could be because of a poor setup.
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As for how much I ride ... it depends. More in summer, less in winter. Over the past 24 years: as much as 14,000 km one year and as little as 2000 km ... depending on life, health, work, school, etc. etc. And as for individual ride distance: anywhere from tiny round the block rides ... to 1200K randonnees.