How much is too much? (diminishing returns)

Since you already have a running background, your best focus is on your swimming and biking.

I would look at the available sprint plans here, and pick a plan that is weighted more towards your weakness ( swimming and/or biking ).  You can probably increase the time in the plan for running, since you're doing longer than sprint distance runs already, but 2-3 runs per week should be more than enough to keep your current level of run fitness, assuming you're able to swim and bike 3-4 times a week ( each! ).

Where you will gain the most is from increasing the frequency that you swim and bike each week.  It's easy to train in the area you enjoy the most/are better at.  To improve though, you need to focus on your weaknesses and bring those up to the same level.

Where do you stand today in terms of swimming and biking, can you give us an idea on your experience/fitness in those areas? 

With swimming in particular, in my opinion you're definitely well served by doing 2x-3x sprint distance ( so swim sessions of 1800-2400 meters ).  If you swimming shorter than that, you're likely not spending enough time in the water to make gains on your form.  If you are new(ish) to swimming, then your primary focus should be on form and drills, rather than speed work.  When you are comfortably swimming 2000+ meters in a swim session, then it may be worth looking at doing interval/speed work.  Though your main goal is to get time in the water, and work on form.

With biking, I'd say it's similar, though probably a little less "overmileage" than swimming.  Just ride.  You should have good cardio fitness from the running you've done.

With both, the key to seeing improvements, if you are "new" to tri training, is consistency ( 3-4 workouts per week per sport you want to improve ), and being able to do that for a long time.  Increase your durations by 10%.

You need to build a solid base before you really will gain much benefit from intervals/speed work.  The way to do that is consistent, frequent, training.

Besides, once you get a few more sprints under your belt, you'll probably feel the urge to take on the olympic distance, so you'll continue to increase your training for that.   

-Chris 

Maybe on the bike, perhaps.  But I'd definitely disagree with doing that for swimming, based on what the original poster related as his current swim abilities. 

Since he's already strong in running ( compared to swimming and biking ) the focus should be on swimming and biking, and doing only a 'maintenance' level of running.  Certainly his base is there with marathon training on the run.  But it's not there on the swim or bike ( yet ).

When swimming is consistently around or a little under a 2:00 / 100 M pace for a longer set ( 400+ ), then it might be time to start working on intervals/speed.  Up to then, the best gains will come from a focus on swimming form, rather than speed.  Speed will come through efficiency as you improve your form.  Once you've hit the limit of improvement through form, is when you should start working on endurance/speed/strength.

I'm not a swim coach, so I'd defer to anyone with more experience in this area.   

This might sound as a vague advice but you don’t provide a lot info to offer a better one Anyway, I would do maintenance runs and focus on biking and swimming to improve endurance and technique. (in fact is what I've been doing for my own training)

You already have the running base and the extra time on the other sports will benefit your run as well. As you get close to the race you can add some speed/tempo runs to keep you sharp and remind your legs of speed. Finally the better prepare you are for biking & swimming the better you will be able to use your great base and post a better run…

For the swim, I'd shoot for 2-3x the distance (over an entire swim session).  For the bike I'd shoot for 2x the distance.  These are for your 'long' workout.  Every workout doesn't have to be that distance, remember.  And that's what you work *towards* not, where you should be starting your plan.

Frequency is important in order to improve.  Even if it's just a 30 minute ride or swim, if you can do them multiple times through the week, you'll be better served than if you try to do just one monster session on the weekend.

IE you don't need to get out and bike 4 hours on the weekend.  In fact, I think you'd be better served if you biked 1 hour / 4 days a week.

-C 

How about 4 hours on the weekend  AND 1 hour / 4 days a week?

I'm not expert and the time and distance you go really depends on your goals.  Last year when I was just starting out and my primary goal was to finish the race, secondary goal was to lose weight.  I would do about 2x the distance of my race.  This meant that in the pool I was swimming 40 minutes on a long day, though I did do 1 or 2 20 minute swim days each week as well.  On the bike my long ride was 60-90 minutes, with shorter rides of 30-50 minutes.  My run was where I screwed up the most, I would run 3x a week, 5 miles was the long and then two 3 mile runs.

Currently the plan is to build to about the same levels in the swim and bike, but I've been putting a focus on the run, my long runs are between 10-12 miles now with 2-3 shorter runs from about 40-60 minutes.  I end up with about 25-30 miles per week, because I gnerally miss one of my runs due to family commitments.  My long run should be shorter in reality but it's a good time for me to build my overall condition and I run about 80%-90% with a Zone 1 Heart Rate and the rest in a low Zone 2 region.

I started to focus on the run over the winter and slowly built my time and mileage up to that point.  It has helped, last August I could run a 5km in about 30 minutes, now I can run 21-22 minutes.  Currently my goals are to improve overall and since my run was the worst event compared to others at my races I started with that.  At some point in the future I'll work on the other sports as well.

Chris.

If you're not time limited, and can do both, sure.  :D

2-3x are rough guides, if you're aiming at sprint distance.  That's not chisled in stone, as everyone has different abilities and fitness and goals.

By maintenance on the run, I'm referring to only running 2x a week or so.  If you are already doing marathons, you don't need to run 4x / wk to prepare for a sprint.  You need to focus on the bike and swim.

Treat each sport independantly.  If you have a base in running, do speed work.  But make sure you focus on your weaknesses.  Running is not a weakness if you are doing marathons.  Time is better spent improving bike and swim, than on speed work for the run if that's the case.