10K this past weekend, so so results...

Usually it means you raced hard on a tough course.

If the soreness is acute (like a sharp pain), I'd be more concerned about a possible injury.  If it's just aching muscles, then I would be satisfied that you gave a good, hard effort.  I would also look at ways to improve my fitness so I could do even better next time, and not be as sore afterwards.  Don't worry, it takes time, and practice, but you will get there if you stick with it.

It's your level of fitness.  I run the same very hilly 10k every year here on Long Island (Cow Harbor 10k)  and the first year (2006) I ran a 51:XX and my legs hurt for days!  My calves and quads ached...  but I only had 10 months of running in my legs. 

I continued running, lost 15 lbs,  and the follwoing year I ran a 44:07.  My calves and quads hurt afterwards but not as bad.  Then last year, another year of running, and I squeaked in at 43:54 and the following day my legs felt a tad sore, but I was able to get a 28-mile bike and 4-mile run in that following day. 

Fast forward to this year, another year of running and tri-training, and I ran a 42:38!  And the following day my legs are maybe 3/10 sore.  Why?  Increased fitness.  If I took pictures of my legs from 2006 and compared them side-by-side to my legs today, they'd look like entirely different limbs. 

Keep running, training and you will get faster and you will get less sore.  At least that has been my experience