One of the best things you can do in order to prepare yourself well for a triathlon is to incorporate transition runs and bricks into your training. They are both combination workouts that include a bike and a run back to back. A transition run is a short (not more than 10 minutes) run off the bike, whereas a brick usually involves a little longer run.
Even for a strong runner, coming off the bike and starting to run can be difficult because the legs feel “weird” and heavy. In order to train my legs to switch as fast and as efficiently as possible between the two sports, I try to run off the bike as often as possible. After (almost) every ride, I put on my running shoes and head out for a hard 10 minute run. If I am short on time (which I usually am), I will cut my bike ride a few minutes short and still squeeze in a run. If you think ahead and have your running shoes (plus socks, visor, etc.) ready to go, you can practice your transitions at the same time!
I usually do a single brick once a week. Since I will be doing the Olympic distance at the Cache Valley Tri, my brick has a 20-25 mile bike and then a 3-4 mile run*. I push on the bike, but the run needs to be the harder part of the workout. I want my body to get used to running fast right as I get off the bike. In order to maximize this training effect, I also do double bricks, especially 5-8 days before a race. In this case I bike 10-12 miles, run 2 miles*, and then repeat both. This is my favorite triathlon-specific workout. It takes some planning but it is definitely worth it. Go try it out and have fun with it!
*If you are training for a Sprint triathlon, you can simply cut the distances in half.