Gosh Luiz, not having done the Cape Epic I am totally taking a shot in the dark here. It seems like the events are anywhere from 5h to almost 7h for the women professionals... so likely that range give or take two hours for competitive amateur men. Long rides would be on the menu to prepare for that... probably not much more than 6h, you can save the really, really long rides for the race, but some solid back to back 5-7 hour days on weekends would help you prepare.

I would avoid speedwork while you do your long rides. Mixing heavy volume with intensity is a surefire way to get injured or overtrained. Instead, I would spend the bulk of the winter preparing for the volume. You can train at a reasonable intensity when doing volume... up to about 85% but no more than that. No speed, just high end aerobic. In the three weeks before the race, I would start to cut down on the overall volume, getting maybe one very long ride in per week, and add some speed work to help you taper and feel fast on your bike. Basically, you would have the race fall right at the end of a strong aerobic winter training block and then you would end up very strong for the coming XTERRA season.