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).
Id 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 wouldnt 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.