Doing cardio workouts will improve important aspect of your fitness. It is possible to get cardio effect from weight training, as we've seen in the sample circuit workouts in each kit chapter. But if you really want to improve your cardio ability and enter races, you could benefits from buying a cardio machine.
The main types of cardio machines are treadmills, exercise bike or spin bike and rowing machines. You can of course run and cycle outside and both of those activities are fantastic at improving your cardio fitness but a machine is a convenient weather-immune option and unless you live near a road and have access to bike an indoor exercise bike is also problaby going to be more convenient.
The quality of home multigyms has improved enormously in recent years.The best ones now mimic the movements you make when using freeweights, whic allows you to push yourself without getting trapped under a heavy barbel. They're also increasingly space efficient.
The cheaper option will lock you into fixed planes of motions, so it's the machine rather than your muscles thats controls a large part of the movement. More shopisticated (and, therefore, expensive) machines may either have cables wheres movement patterns aren't fixed or the option to switch between fixed path and freeweights.