Training for a Duathlon With a Triathlon Plan?

Sure, just make sure you do bricks both ways, and practice your 5k paces on either end, they'll be different and neither should be at your best standalone 5k pace. Aim to do the first one about 5% slower, nail the bike close to threshold, then pace the 2nd one 7-10% below standalone 5k pace.

Practice it in training and you'll get a better feel for best paces with more race experience

How many weekly workouts are you completing each week? The only reason I would drop the swim workouts from a training plan would be if I dropped tri entirely & was only focusing on du's. Otherwise keep the swims.

Great advice. Thanks!

In only my first full year of multisport I ended up doing more du's than tri's. Just worked out that way. In fact, I find the run bike run more challenging. I trained the same for both, except, once I realized that the final run felt a lot different than run in a tri I made sure I did a few bricks to acclimate to the feeling. For me at least, my body (even more so my mind) is so happy to get off the feet after a first hard run then onto the bike, only to have to run again, is a mental challenge.

The vast majority of my races are Du's (although once in a while I do my best to drown). There's a lot of similarities in how you train, but also some key differences. First, you need to practice the transition from run to bike. After a hard run, get on the bike (as soon as you can after finishing the run, preferably within a few minutes) and do a couple of miles on the bike. Take it a bit easier the first mile, then push the second. You'll learn pretty quick how much recover you will need during the first mile and how hard you can push the second. This knowledge is crucial in a race.

Second, you do spend quite a bit of time running in a duathlon so you want to train that more, but to swap all your swim for running, in addition to the running you would have in the training plan may be more than your body can handle. Most duathlons tend to have shorter runs, so after you build a solid base, you will need speed work.

Third, duathlons are more painful than triathlons (certain there's going to be disagreements here, but in my opinion, they are), you will need to learn how to suffer. Great training for this is free standing 5k's and TT's.