Yes and no.

No..you do not have to concentrate on one distance and specifically train for that distance (with the exception of a marathon...that you need to devote yourself to).

Yes...you can train for all of them and improve on your times for all of them.

The key is to do different kinds of runs each week.

Now...some people can take this and make it very complicated and tell you which track distances you need to improve and blah, blah, blah.

But, let me try and explain it in English.

In order to be ready to run any distance (from the 5K to the 1/2) on any given weekend, you have a solid running base that is made up of those distances.

So, incorporate one of these runs each week -- do a short, fast run (3 to 4 miles), one or two days of just-less-than-10K-pace runs (5 to 6 miles) and a slower, long run on weekends (8 to 10 miles).

The key to improving your running performance is to NOT get stuck doing the same run at the same pace for the same distance for ALL of your runs. It's fine to have a basic loop that you do two or three times a week, but you should strive to throw in at least one shorter, faster run and a longer, slower run each week, too.

One other thing that is definitely worth tossing into the mix that will show you BIG improvements is running hills. Running hills will give you power...and confidence when you meet up with a hill in a race (when everyone else groans, you'll power up them).

Some other things that will improve your running is to do some cross training -- particularly anything that has to do with flexibility, core strength and weight training.

Can't fit them all in...don't sweat it...just keep your body guessing and it has to work hard to keep up. Don't get caught in a rut.

Due to circumstances beyond my control, I've had to catch-as-catch-can any YMCA fitness classes. I never know which days I can make it to which classes...I have class schedules for all three YMCA's at my desk & dash off to a class any chance I get. Sometimes Pilates, sometimes ExerBall, sometimes BodyPump, tonight...Step/Core Strengthening.

You know what? I've seen a major improvement in my muscle tone and I'm certain it's because of my helter-skelter gym class schedule and the variety of classes I'm taking. My body never knows what I have in store for it. I have to tell you...I took Body Pump for SIX years...twice a week...religiously. Only after I quit and started my new whatever-I-can-get-to schedule, have I got the biceps I tried to build for SIX years.

My basic fitness routine is this:
Two to Three times a week -- a 5 to 6-mile run at about a 10K pace (usually run with others)
Once a week -- a shorter 3-4-mile run at 5K pace (usually by myself)
Once a week -- an 8-mile hill run at just over 10K pace
Once a week -- a 10 mile run at 1/2 marathon pace

NOW....all that SAID...there is value in training for your specific goals -- particularly if you're the type who needs the motivation of a specific goal. Runner's World magazine had a great article about this in the May issue.

One last thing -- it's very important to take rest days (I always have one or two days I don't run...I may go to fitness class, but I don't run). AND...if at any point, your body is telling you it needs a break -- give it a break. Two days off to rest or let a little injury properly heal is better than two MONTHS off when you run on a sore leg that causes you to lean which causes an ache in your back which causes you to pull a hamstring or you end up with a stress fracture, etc., etc.