If you want to improve power, typically doing a max effort at that duration and then doing repeats of 95% or so of that power is a decent enough start to get you going. With heartrate, hitting a corresponding hr should do the trick as well. The shorter and harder the effort, the longer the recovery.

Speed is a separate issue from power. Generally I focus on power and then start transitioning to getting that power to equal speed. Position is one of the biggest factors in that regard, but equipment follows quickly after. Depending on your current setup, latex tubes with fast tires could net you a 1/2 to 1 mph increase. It's a massive difference and it's relatively cheap speed.

Muscular endurance comes from training more, longer. The more you train, the more you can train, and the more harder training you can do, which, when coupled with appropriate recovery, is what you need to get faster and stronger.