Congratulations on your first 5K! Sorry to hear your knees are sore. A few things that help me when I have a little pain in my joints after races...

1. Gentle stretching, if you are feeling pain during your stretch, back off.

2. Soak in a warm tub with some Epsom salts.

3. Make sure you are hydrating. Sometimes we get body aches when we are dehydrated.

4. Apply ice to area in the first 24 hours, 20 minutes on, 20 minutes off, alternating on/off.

5. Gently massage on a topical ex: Tiger Balm or BioFreeze to the sore area.

6. As for going to work in the morning... keep moving but not too fast. If yo sit still or stay stationary your muscles may tighten up.

7. No running for 2-3 days, gentle walking as long as you do not have pain.

A little soreness is expected. But, it will pass. You'll be out and running soon. Keep up the good work!

I've been running for about 10 years and I find that Bananas and Gatorade are very useful.

After a big race, the second day can be the worst.  So, the most important thing I have found, is to do a light jog (~15min or so) the day after.  To me, the soreness isn't what gets ya, it's the tightness.  Hang in there, and congratulations on completing your first 5K!