First up, is professional triathlete, Justin Park. He shares a favorite swim, bike, and run workout. Proceed with caution!
This one takes some serious focus, but it's one that will give you a VERY clear indication of where your swim fitness is and how you will perform on race day. Warm up 1-1.5K with a mix of easy swimming, drills and kicking. Finished with a few shorter, sharper efforts (25s or 50s) to get ready for the main set.
The goal is to really push the opening 200 to mimic the hectic (and usually fast) nature of the start of a swim, but then still be able to settle in at a strong, solid pace. The goal for the 300s is to equal split all of them (each 300 within 1-2 seconds of each other). I repeat this workout every 3 weeks (off of an easier day before to really be fresh and push it) in the final 15 weeks leading into my peak race of the season; however, it can be used year round simply as a practice of pace change and even splits.
This workout can be done within a large variation of ride duration (anything between 2-5 hours). It's a blend of strength and aerobic threshold work, and it truly builds the requisite durability to not only ride well but also run strong off the bike.
The goal is to hold the same wattage over a low cadence and something that more closely mimics race cadence. The transition from low-to-high cadence will definitely take some practice, but the body does adapt. You can also start at shorter intervals (e.g., 5 x 8 minutes as 4/4) and increase the interval length by 2 minutes each successive week.