Best way to improve swimming distance or time?

I would bet you are trying to swim too fast.

Definitely get someone to look at your stroke if you can.  Even if it's just another experience swimmer at the pool.  One or two small tweaks can make all the difference in the world!

And yes...keep at it.  Swimming takes a lot of consistency.  Get there 4-5 times a week if you can! 

I've been workign with a coach at the my local Y and the last session he had me really slow my stroke down and stretch out my stroke and I was amazed at the difference in breathing, I was able to swim much longer yardage without stopping and still had my breath. 

After you have someone look at your stroke:

http://ruthkazez.com/ZeroTo1mile.html

Definitely stick with it!

I went from not having done any real swimming in December of '05 to finishing a half ironman swim in June of '06.  It wasn't pretty, and I breast stroked a lot, but I made it without being completely in the back of the pack (but not far off).

I spent a LOT of time in the pool this year (comparatively) and am finally having a one on one session with a coach tomorrow for the first time.

Congrats on your progress and welcome to BT!

Make sure you are giving yourself enough time in the pool. Many triathletes that aren't strong swimmers only have 20 or 30 minute pool sessions, hardly enough to improve. Give yourself 45 min to an hour even if you are only doing 25's and 50's. Work a lot of 25 yd drills, do a lot of 25 kicks, anything to keep it interesting.

There is one good thing about not being an efficient swimmer, you are getting a much better cardio workout than those fast swimmers!

PM me with some of your pool workouts and I'd be happy to offer some suggestions or workouts that would help you.

If you hit the pool 3x per week, you should easily - EASILY - get up to 750 yards continuous swim in 3 months.

First, have someone observe your stroke to see if there are any obvious changes you need to make to your stroke. Good form makes the swim easy and, therefore, faster but bear this in mind: MANY/MOST/THE MAJORITY of people at triathlons do not have great form. That tells you that you don't have to have perfect/great form to get through the swim. I saw an elite athlete with an atrocious windmill stroke at St Anthony's Olympic Triathlon - he was 3rd out of the water. He had a fast, crappy stroke. Go figure. I recall an Olympic swimmer who was delicately asked to describe her stroke (it was unusual, shall we say) and her reply was "I would describe it as rapid." Can't argue with results. Anyhoo, I don't recommend cultivating a sloppy stroke, just pointing out that it is not the end all/be all unless you want to be an elite athlete and even then, apparently, it is not an absolute requirement. Also, what you lack in form you can somewhat compensate for  with a strong core (I've seen a beginner with a positively horrendous stroke swim at better than most beginner times thanks to his strong core.)

Second, just swim.  Decide that you will put in 200 yards on Monday. If you have to do 8x25 to get the 200 yards, that's ok. The next time, do 4x50. Then the next time do 6x50. The next time do 6x75. Then 6x100. Then 6x125. And so on. You could probably progress much more quickly than that but I'm using conservative incremental increases so that you can get there easily.

If I were you, I would also spend some time at the US Masters Swimming website. Plenty of articles on swim form, building your own practice sets, etc., and a discussion forum. You could also use that site to find a local Masters team to join.

Swimming is such an awesome and self-rewarding sport. I hope you enjoy it. Good luck!

DISCLAIMER: I have a beautiful stroke but I learned it at a very young age; it's just a matter of muscle memory for me. Maybe I wouldn't have this perspective if I had a crappy stroke, so take it with a grain of salt. In fact, I would advise that you take ALL internet advice with a grain of salt - everyone has a POV that might be totally irrelevant to your goals and objectives. However, I assume that if you were intensely serious about becoming a competitor that you would be seeking the advice and input of a real life coach instead of asking for advice on a beginner's board.