For those of you that are time strapped and willing to push yourself super hard, then grab a stationary bike for a round of tabata. I try to hit it twice a week and there's nothing more powerful for me in the area of speed and endurance. It's a four minute workout- 8 reps of 20 secs max effort followed by 10 secs rest. When I'm done I'm just as tired as I am after a seven mile trail run. It's not for the faint of heart, because done properly you will be at the edge of seeing stars But it's absolutely amazing and, as far as I know, the only exercise that can improve speed and endurance. You can ease yourself into it by adjusting the resistance of the bike. Here's a link. People try it with push ups and setups, but that's trash. The original research was done using stationary bikes. It will change your body and energy levels pretty quickly. Here's a link. http://www.tabataprotocol.com
During the other three days I try to get in trail running and olympic style lifts where the weight is moving from the ground to over my head. I'm a little older- 42- but this seems to keep me pretty healthy and quick.