As I was going through the C25K program I experienced the same thing and posted the same question.  The best answer I got, and now experience proves the answer correct, is that as you add speed or distance, the soreness is normal.  When you get to a point where you are maintaining, it will decrease significantly and eventually dissapear until you push yourself again.  This program is a progressive program so it is normal.

My experience is somewhat different.  I am about to turn 62 and started running at age 52 so I have been running for 10 years give or take.  I have maintained about 6 miles a day 5 days a week for the last 6-8 years (I didn't start record keeping immediately as I didn't know I would keep it up).  After all that running my legs stay "tight" all the time.  I don't know if this qualifies as fatigue or not but it is not really pain just tight.  I recently (couple of years) added cycling as cross training but the extra miles on the bike has not relieved the sensation.  But I find it amusing that I have a hard time getting out of the recliner at night to walk across the floor but can still run my six miles.  Go figure.

Have you ever tried an "ice" bath? It really works wonders after a long run. Just fill up the tub with cold water. I add a 20 pound bag of ice. I get into the tub, enough water to cover your legs then rip open the bag of ice. It takes about 30 secs to get over the shock of the coldness but then after that you don't even know your in cold water. I try to stay in at least 15 minutes. Then I drain the tub and stand up. New blood goes rushing into your legs and it feels great!

You do have to be a little cautious because some people can not tolerate this cold of water.

Check your diet.  Make sure you are getting all the vitamins and minerals you need.  Electrolytes (sodium, potassium, etc.) are particularly important.

Check out some online training plans for the event(s) you're interested in.  Active has training advice in its general running section (http://www.coolrunning.com/engine/2/index.shtml), as do other sites like www.halhigdon.com and others.  You should at least be alternating hard workouts with easy/rest/cross-training.

And don't forget stretching.  But make sure your muscles are warm before stretching.  Stretch after your workout.

Len

I used to get more muscle soreness when I was younger.  The effect of trying to power my running.  I'm now 58 and focus more and more on staying relaxed (it is hard to hurt a muscle that is relaxed).  Try focusing on your alignment (posture) keeping yourself from collapsing.  The more you collapse the more your muscles have to work to hold you up and therefore the more sore they will become.  Also, what really helped me relax my lower legs (I did this a lot in the 50 mile race I did at age 55) is to just think about "lifting" my ankles - i.e. not pushing off.  The day after the race my legs were not very sore - my obliques were