If I'm not sore, does it mean I'm not working hard enough?

I am pretty much in a constant state of soreness these days, but I am also following a half marathon training program for my first such race in September, hence running longer/faster/up more hills than I ever have before. Not to mention the weights & core work

I find I generally will get sore if I increase my work outs, but its not unbearable soreness like what I used to get when I was unfit. Its just a constant mild discomfort in the background now

For running, soreness is usually associated with lactic acid.  That is, left over lactic acid in muscles following a work out makes them sore.

With a heart-rate monitor, one can a pretty accurately guess as to where one's "lactic threshold" (LT) is in regards to running.  This threshold is the number of beats per minute where your body is producing more lactic acid than it can remove.  Over the past few months your body's ability to remove lactic acid has improved and, therefore, you don't feel sore as often.

The only time you should worry about pushing to a point where you feel sore is when your training calls for it.  I'm also training for a half-mary and I only have maybe one or two LT runs per month until I get closer to the race.  When a program calls for a "hard" run, that's when you should push to the point of soreness.

Over the past two years, the only time I am ever really sore is after a race (even through HIM training). I am in IM training now and do have some soreness, but I only ever notice it when I run. A lot of that has to do with the fact I was injured this year and didn't run for a long time, so these long runs are just a little too much stress for the legs and recovery takes a while. So, nope, don't worry about the lack of soreness.