I would say at this stage distance rather than time, however, I wouldn't confine yourself to pre-conceived increments; listen to your body. Only you know your fitness history. If you are in recovery mode after a health concern or have never followed an exercises regime, your progression should be more cautious than pushing limits to the point of soreness, or breathlessness. Keep in mind that rest is required for recovery. Your intro reads as if you are looking for the fast track for your goals, and as counterintuitive as this sounds, this requires time set aside for days off the bike. If you are healthy and aware of your fitness capabilities from another physical activity, the greatest/rapid gains will come after pushing to the point of failure and adequate time off for reparation. Of course each of us are different, you may have external muscles which will progress more rapidly than cardio pulmonary capacity, or plenty of breath but no legs, but in time they will come together.