You will want to limit your anerobic workouts. Your recovery from these take twice as long but you won't feel it until nearly 48hrs later. If you are not recovering properly, you'll drag yourself down in time, get sick and not be able to train at all.
It is important to get some muscular endurance built into your training that isn't so cardio intense - like weight training at a gym if possible. There are other gym free methods too - so whatever works for you.
If you want to challenge yourself HR-wise, do activities on off days that are not on a bike - like aerobics or swimming or running. This is the best way to challenge your muscular system and HR because it doesn't build the monotony that only biking will do.
For a century ride, you are going to need strength in your legs - as well as in your lower back and stronger abs. You can work on these in an isolated manner that will all benefit your riding.
Don't rely on your HR monitor alone. It will not tell you that your legs are giving out.
