Realistic times at my age.

You'll make better gains swimming then you will running.  At 5'10 135 and 3.5% BF it's not like you're going to drop a bunch of weight to gain time......and your swim time is slow as hell. (comparative to your run) Go there for the fastest gains.

See that bolded part?  You won't improve much with that volume......it's a simple fact.  BUT.....you can stay where you are if you stick with it.  A sprint tri is not a big hill for a guy who is in shape like your are.  Real gains come with real work.  2 or 3 runs and 1 or 2 swims won't get you much.

I'm not trying to discourage you.....in fact, I'm encouraging you to do what you can through Fall.....you'll be fine.  Your gains will come when you have more time.  A realistic possibilty now, as you asked, is to maintain where you are. If you have any gains at all it will be minimal.....which is not a bad thing....it's what you have time for....not a damn thing wrong with that.

I'm a bit educated on the subject.  I had a college track "career", first tri in the mid 80's.....my son has an elite tri card and is a college runner......31:30 10K....15:something 5k.....4:10 mile.....54 minute sprint tri....can swim an 8 minute 800 with very little training because he was a childhood swimmer, which you are not...and that's a BIG deal for swimmers.  You don't know how to swim, and you won't learn on 1 or 2 swims per week.  Again, it sounds ignorant, and I'm sorry,  but it's just honest.  (I'm taking some liberties because you are obviously a fit guy,  so I figure you want to hear realistic advice)

It's A LOT of work to get faster......nobody here will tell you different.  That's why they are proud of their gains.  They either had the time, or they made the time, to get faster.  It is what it is.

Disregarding the body metrics, current training constraints, current fitness, etc. and just focusing on the "AT MY AGE".  I think this should be enlightening. 

Timothy O’Donnel born 1980 (40)

Matt Russell born 1983 (37)

Andy Potts born 1976 (43)

They were the top three American Pros at Ironman Kona in October.

The M35-39 and M40-44 are about the most competitive age groups in Triathlon.  I have done two Triathlons in the last month.  Both races had the top two finishers from the M40-44 Age Group.  At age 38 you are in your Triathlon prime.  Age is not holding you back, but it does take about 3 years to start to peak in endurance sports so the guys that started at age 30 are going to be a lot faster at age 38 than the guys that started at age 36 are going to be at 38.   

P.S.  I did 1/2's my first four years of Triathlon and just did my first two sprints in the past month (and will do my 3rd sprint in August).  I am in the M40-44 AG and haven't slowed down yet.  I have done 9 Triathlons and have PR'd in every race.  :-)

If you are doing a 1hr 38min Sprint now then your goal should be to improve on that.  You should see a lot of gains in the first three years.  After you peak the gains will be smaller.  I saw gains in the range of about 10% a year for my first 3 years.  Now if I can shave 1-2% off I am happy and if I can shave 3% off I am doing backflips.

At age 38 you could go for 10 years without slowing down.  Endurance sports are not like other sports.  In some sports, if you see a 20 something guy and a 40 something guy going neck and neck to the finish line the 20 something guy is where you are going to put your money but in endurance sports that flips to where you are betting on the older guy to win. 

Young people muscle their way through things.  Old people finesse their way through things.  Finesse wins out in endurance races when you have two people who are the same speed. 

You have plenty of years to grow.  Your age is not a factor.  I have been improved in every race for 5 straight years and I am in my 40's. At this point I don't see my age slowing me down at any time in the near future. 

Welcome Jeff,

If you haven't experienced it yet, you'll see top race finishers in their 40's regularly (5K's and triathlons).  Some of this is because the fastest young guys (like LB's son) run for colleges and therefore not in typical open road races.  You're not too old to get faster, but...you won't get faster without changing your training approach.  

To start, you've got a good aerobic engine and you're not carrying around extra baggage.  You're overall potential is HUGE (swimming, biking, and running).  

You're probably experiencing "sinking legs" in the pool.  With no body fat and nobody providing real-time feedback to get flat in the water, you're likely to be frustrated with swimming gains.  I've been well over 3.5% body fat and while I never swam competitively, I was a lifeguard and swim instructor.  I'd usually swim somewhere around 2:00/100 either in continuous swims or more structured workouts.   I could do 100 yards in 1:45, rest 0:05-0:15, and do it again.  I was typically swimming 2X/week averaging 2000 yards per swim.  Two different times, I got involved in more structured swim training.  At my peak, I could swim 100 yards in <1:30 (not at all fast by swimmer standards) and repeat with 10 second rest.  The one time, we met on Saturdays and swam 3000-4500 yards.  The other, we met 3X per week and I averaged 2500-3000 per swim.  

Is it possible/realistic to drop 2 minutes on your 5K and get down to 1:30/100 on the swim?  Yes.  But it'll probably take more than 2X/week workouts.  

IMO, "realistic goals" is a euphemism for "safe goals" and my experience is that relatively little "push" occurs when you live this way.  I wouldn't say 12 min/800 is out of reach, but to reach it, you'll have to do things a lot differently to achieve it.  As such, I'd say it's an "aggressive goal."  

My take: if those benchmarks were recent and you are smart and consistent in your training, you have the potential to get faster until your late 40's.  The trick (consistent training with few injuries).

Swim: 1:20 pace is tough for adult onset swimmers.  You can get there but you will need good instruction and yards.  4-5 swims a week in the range of 10-15k yards.  3 or less swims a week and you're just maintaining what you have for the most part.  At 2:00/100 pace you need stroke development and swim fitness.  Get some 1 on 1 coaching, join a masters group and swim more.

Bike: time in the saddle by itself will get you gains.  Equipment and position optimization will get you some gains too, but it really boils down to building a bigger engine.  TrainerRoad or Zwift workouts will give you some structure.  Otherwise, just ride with people that are faster than you.

Run: I think you can get to 19:xx but you will need to run on a pretty consistent basis to do that.  1-3 runs a week isn't going to cut it.  You'll get faster just by running consistently 5-6 times a week at an easy pace.  Even then, you'll probably need a run focused block (with some race-pace training) to get down to your goal.