There are generally two types of athletes out there; those who are single-sport devotees and those who want to be ready for everything. And for the record, even though there are three disciplines in triathlon and two in duathlon, I still consider anyone who calls themselves a triathlete or duathlete a single-sport devotee. They have made a choice and chosen a label. The generalists are those who want to be fit enough to handle whatever adventure comes their way, and that sounds like you.
Balance is the key to being a generalist. If you want to be able to participate in a wide range of sports and activities, you should incorporate a wide range of activities into your training. If you never ride a mountain bike, youre not going to be very proficient when you hop on one. You might have the aerobic engine you need, but if your muscles and joints aren't ready for the body position and range of motion, youre not going to have much fun out there.
Specificity works both ways. Single-sport devotees rely on specificity to excel as runners or cyclists or skiers. For the generalist, relying too heavily on any one sport or activity to develop fitness would inadvertently apply the concept of specificity to their training. Be careful, you too could end up a runner (just kidding, but you get the point).
I'd venture to say that the optimal combination of activities for you would be two to three runs and two to three rides each week. Within those activities, divide the runs into rolling trail runs and hilly, technical trail runs (if available). If you can, ride a road bike once a week, and a mountain bike twice. It wouldn't hurt to substitute some form of bouldering, rock climbing, yoga, swimming, or racquet sports (squash, racquetball) for at least one workout each week as well.