How can I build my endurance back up?

I'm a boxer and I got sick with the flu for about ten days. I am still getting over it, but now I'm just a bit congested. When I was sick I didn't work out at all I just kind of laid around and slept. I went worked out and got in the ring today and I feel like I lost a ton of muscle m and I can't last long fighting or hitting the bag anymore. I'm going to start running tomorrow to build back my endurance so I can last a while and lifting weights. Is there anything else I can do to build up my endurance and strenght fast? I feel like being sick took all my stamina away and I really need it. How can I regain it fast and build it up even more?