I just got my butt kicked in a way I've never experienced before, and i'm not sure whats going on, or how to fix it.

I went out for a ride, after not having ridden at all for basically a week. My normal rides aren't particularly long, an hour and a half, but very high intensity(i go all out, 30-35km/h range). And I've been doing that anywhere between 4 to 5 times a week. Bunch of stuff + weather came up last week, and I took an entire week off, I figured if anything that would give my system time to recover/heal.

So I finally get out today, and not more then 5km into my ride, my legs shut right down(cramped up). I was barely in warmup mode, no effort going on, my ride starts on a slight downhill. 5km in, just cruising in what would be considered a brisk walk effort wise, my quads cramped? I kept the ride going, figuring it would clear up/out, but it wasn't long before I realized it wasn't happening at all, I cut my ride short to a whopping 20km, and came on home. The cramping/pain came and went throughout the ride, there were times were I could output some power and it felt "decent" by the end. But after I got off the bike I've never had my quads this sore in my life, it literally feels like someone took a healthy swing at my quads with a baseball bat.

Is this lactic acid? did I strain the muscles somehow? I could understand that if I took off out the door mashing away full force or something. But I never even got started. Oddly enough, on the ride, It was substantially easier to push a big gear, running my cadence up actually brought the cramping on in a hurry(which seems completely counter intuitive). What do I do to fix this/help sooth it? Do I go out and spin a ride or two? do I mash a ride or two? Do I lay off some more(I've never taken an entire week off before), or do I ride as much as possible? Do I take hot baths? or cold showers?

I've never had anything like this happen before(and certainly never been this sore after zero effort), no idea what is going on here, or how to correct it.
Thoughts?