generally you want to increase mileage slowly (like 5-10% a week). it might be better to think in terms of time. as in, don't increase riding time by more than an hour or two per week. the intensity doesn't need to be too high at this point.

after a few weeks, don't add anymore. let your body catch up to what you've worked up to. stay at that level for a while, then start to add more.