I've raced Boulder in 2014 and 2017 and have spectated the remainder of the years. While the bike course changes every year (and now the run course has changed) the thematic elements from 2015 forward have generally remained the same. Here's my thoughts on training for Ironman Boulder (at least in it's present form in 2018....)
Swim
For those who live at lower elevation, you'll mostly feel the affects in the swim, since that's when you're most oxygen deprived. I like having my athletes do breath control workouts in the pool to help get used to not having as much oxygen as you'd prefer. A good set is for a warm up or cool down and do a 200 or 300 where you breathe every 3 strokes / 5 strokes / 7 strokes etc per 50 yards. I'm also a big proponent of breathing every 2 strokes during normal swimming because I like oxygen.
Being that the race is so early in the season, many of us won’t have the opportunity to get in an OWS prior to race day. Most OWS venues in Colorado aren't open until mid-May, and even then, they're bone-chilling. Aim for at least 2 OWS at the full 2.4 mile distance so you can get reacquainted with sighting, wearing your wetsuit, and swimming without a black line beneath you.
Bike
The Boulder course changes every year but generally (except for the first year) the course sticks to Boulder County Roads. This means many rollers and thematic elements. Generally, northbound roads are rollers with a net elevation increase (southbound is the opposite), westbound roads are moderate climbs (and the way the course is set us, the western legs are usually around 4 miles long). Eastbound roads are the reverse with nice downhill stretches to give you a bit of a break. Overall, you don't head in any direction for very long (30 minutes tops) which means you get a nice break in terrain, wind, and scenery. For 2018, the main bike challenges are the hill at St Vrain right before 36 (it's "attention getting") and the slog up Hygiene.
To train for this, if you live in Denver, go and ride the course at least once, just so you're familiar with it. If you're not lucky enough to live in Denver, incorporate some 30 minute harder sustained efforts (maybe 30 min harder / 15 min easier / 30 min pace / 30 min harder) to mimic the terrain changes you'll experience in Boulder.
For your shorter rides (1 to 1.5 hours) do some small (less than 5 min) harder intervals (suggest something like 3-4 repeats of 2 min 80% / 2 min 90% / 2 min recovery) to mimic the hills and to help get that power going under a full peak month training fatigue load. This will help prepare your legs (and your mind) for those rollers on race day during the second loop.
And of course, there's my advice of "ride your bike until you don't want to ride anymore and then ride some more anyways". I usually have my athletes do at least one weekend with 2 bigger back to back rides. This accomplishes two things: really good aerobic conditioning without too much toll on your body and a whole lot of mental training. Getting on that bike the second day is HARD but it will set you up mentally for success on race day for when things are hard.
Run
On paper, the run course doesn't look too bad. There aren't many large hills, it looks mostly flat. What looks flat actually had a bunch of small underpasses (which means some ups and downs) and a net elevation increase from the eastern side of the course to the western side of the course. This means you have an uphill course to the finish line, when you're most tired. You actually do this uphill slog twice... the second time is definitely harder than the first time.
To best prepare for the run, I recommend mimicking the run course as much as possible. For starters, the run course is mostly pavement, so if your body can tolerate it, run on pavement for training. Seek out terrain that is similar to the run course - slight rollers and a long (4-6 miles) gradual uphill at the end of your runs. Doing this will really help to prepare your legs and your mind for the run course challenges.
Run volume is super individual so I won't really give advice on that (hire a coach!) but if you are finding that you're legs are having a hard time with the volume, breaking your runs up into 2 segments on the same day is a good way to approach things. Do the bulk of the run in the morning and save 3-5 miles for before dinner (and make sure you eat and get some rest between runs). This will help prevent the form breakdown that most experience at the end of those long IM training days while still allowing for volume loading.
Swim
For those who live at lower elevation, you'll mostly feel the affects in the swim, since that's when you're most oxygen deprived. I like having my athletes do breath control workouts in the pool to help get used to not having as much oxygen as you'd prefer. A good set is for a warm up or cool down and do a 200 or 300 where you breathe every 3 strokes / 5 strokes / 7 strokes etc per 50 yards. I'm also a big proponent of breathing every 2 strokes during normal swimming because I like oxygen.
Being that the race is so early in the season, many of us won’t have the opportunity to get in an OWS prior to race day. Most OWS venues in Colorado aren't open until mid-May, and even then, they're bone-chilling. Aim for at least 2 OWS at the full 2.4 mile distance so you can get reacquainted with sighting, wearing your wetsuit, and swimming without a black line beneath you.
Bike
The Boulder course changes every year but generally (except for the first year) the course sticks to Boulder County Roads. This means many rollers and thematic elements. Generally, northbound roads are rollers with a net elevation increase (southbound is the opposite), westbound roads are moderate climbs (and the way the course is set us, the western legs are usually around 4 miles long). Eastbound roads are the reverse with nice downhill stretches to give you a bit of a break. Overall, you don't head in any direction for very long (30 minutes tops) which means you get a nice break in terrain, wind, and scenery. For 2018, the main bike challenges are the hill at St Vrain right before 36 (it's "attention getting") and the slog up Hygiene.
To train for this, if you live in Denver, go and ride the course at least once, just so you're familiar with it. If you're not lucky enough to live in Denver, incorporate some 30 minute harder sustained efforts (maybe 30 min harder / 15 min easier / 30 min pace / 30 min harder) to mimic the terrain changes you'll experience in Boulder.
For your shorter rides (1 to 1.5 hours) do some small (less than 5 min) harder intervals (suggest something like 3-4 repeats of 2 min 80% / 2 min 90% / 2 min recovery) to mimic the hills and to help get that power going under a full peak month training fatigue load. This will help prepare your legs (and your mind) for those rollers on race day during the second loop.
And of course, there's my advice of "ride your bike until you don't want to ride anymore and then ride some more anyways". I usually have my athletes do at least one weekend with 2 bigger back to back rides. This accomplishes two things: really good aerobic conditioning without too much toll on your body and a whole lot of mental training. Getting on that bike the second day is HARD but it will set you up mentally for success on race day for when things are hard.
Run
On paper, the run course doesn't look too bad. There aren't many large hills, it looks mostly flat. What looks flat actually had a bunch of small underpasses (which means some ups and downs) and a net elevation increase from the eastern side of the course to the western side of the course. This means you have an uphill course to the finish line, when you're most tired. You actually do this uphill slog twice... the second time is definitely harder than the first time.
To best prepare for the run, I recommend mimicking the run course as much as possible. For starters, the run course is mostly pavement, so if your body can tolerate it, run on pavement for training. Seek out terrain that is similar to the run course - slight rollers and a long (4-6 miles) gradual uphill at the end of your runs. Doing this will really help to prepare your legs and your mind for the run course challenges.
Run volume is super individual so I won't really give advice on that (hire a coach!) but if you are finding that you're legs are having a hard time with the volume, breaking your runs up into 2 segments on the same day is a good way to approach things. Do the bulk of the run in the morning and save 3-5 miles for before dinner (and make sure you eat and get some rest between runs). This will help prevent the form breakdown that most experience at the end of those long IM training days while still allowing for volume loading.