Brick sessions are a must, but you’ve picked up on that already. Like others have said, the second run is a killer. I found that doing a bit of (what is now in vogue) “crossfit”, did me the world of good. Basically, burpees, squats, press ups, rowing. You can find plenty of workouts online. This really helped strength in legs for second run. It’s amazing how quickly your body deteriorates after the bike section. Good luck!