I found the coolest toy for sparring training
here. It is a round timer online. You can use it to make sure your workout matches the conditions under which you will be sparring/competing. Ideally, you should be able set the round timer to your round length, and set the rest time in between, and go all out with whatever exercise you're doing for the whole length of the round. Realistically, you can't do this, especially starting out (and most people don't fight this way, either - they go 'all-out' in short bouts and fluries of about 20 to 30 seconds, then move around the ring for shorter rest periods during each round).
Start by reading everything you can about aerobic conditioning, anaerobic conditioning, and the differences and similarities between the two. There is still lots of disagreement, even among the experts, about which is more important for sparring, but everything I've read says that anaerobic exercises will help your performance in both, while aerobic training will only help your aerobic endurance (so even if you have great aerobic capacity, make sure you are doing anaerobic conditioning)! Then pick whatever type of anaerobic exercise you're doing, and set the round timer on your computer. Start with something like interval squats (as many unloaded (body weight) full squats as you can do) in one 20 second period, with a 10 second rest, then repeat 6 times or so. Gradually build up to 30 or 40 seconds.
Also try with 30 second rounds and 30 second 'rest' on your timer, but go for 6 sessions straight of a variety of exercise at max volume (e.g., no rest at all between 30 sec squats, 30 sec burpees, 30 sec skip rope, 30 sec pushups, 30 sec shadow box, 30 sec crunches), then 30 sec rest and repeat, x6.