Pacing is definitely important during any race/ride.
Recently, a lot of research points towards intensity as the best method for building cardio. Doing more intense sprints mixed with mellow spins would be good. For me, running and jogging are awesome ways to cross train. Also, work your core. Many cyclists will buy a 3,000 dollar carbon bike when what they should be doing is working their core muscles. Abs and pecs help keep your bike steady in a long climb. They also keep your spine in place when pushing out of the saddle. Lots to take in, but hopefully this was helpful.