How to stop muscle soreness/stiffness from exercise?

So you know that 10K run I asked about a while back? Well, I finally did it yesterday. I had so much fun and it was awesome! I didn’t even feel out of breath really afterwards (granted I power walked half of it and really wasn’t trying to finish in record time or anything). It was my first time ever doing 6.2 miles (plus an extra 2 miles to walk back to the car). My normal route is only 3, but is mainly consisting of the biggest hill I’ve ever seen in my life, which made it way harder than a milder route that’s 6.2 miles long. That and being around all the people (biggest 10K in the world) made the whole experience awesome. Here’s the issue: I was perfectly fine after the race. I stretched for about 5–10 minutes. After about an hour, the back of my knees got extremely sore/stiff and persisted throughout the day. Went to sleep and got up today, my legs feel awful! Not just the back of my knees hurt now but my entire legs and lower back are killing me. I felt fine after the race, but the next day feeling is getting me. When I sit down for a while I’m fine and don’t feel it, but the moment I try to stretch my legs or stand up I’m in pain. I’m waddling around the office like an old lady and my boss is looking at me weird. I just want to feel better! Please help! A hot shower didn’t work…....hm…..maybe I should convince my boyfriend to give me a massage? lol