I wouldnt worry too much about losing the fitness you have developed in your base phase as long as you are careful not to overdo things later in the year.
It takes far less work to maintain any given level of fitness than it does to gain it in the first place, so during the copmpetition phase I would emphasise really good quality specific sessions and concentrate on recovering really well between them. Several other posters on here seem to have discovered this for themselves when they say that they run better off just two workouts a week with the rest being made up of relatively easy running. The reason they do worse off more frequent sessions is simply that these specific workouts are very stressful and recovery is prolonged.
You are correct to schedule more volume (mileage) during the base phase, as the emphasis here needs to be on volume rather than intensity. However, this is also the time to do more supplementary type work, for example weights, jumping exercises, hill sprints, drills. The exact details will depend on your event, but it is a big mistake to just emphasise low intensity mileage and then expect to run good sessions at race pace later in the year. When you get to the pre-comp phase you are trying to coordinate all of the fitness capacities you developed earlier in the year - not start developing some from scratch. This means that there may actually be a wider range of activities in this part of the year than closer to competition.
As to the length of this phase - the bigger the base then the higher the level of specific training that can be performed and the longer high levels of performance can be maintained. However, as more mature athletes typically have longer training histories than younger athletes, they can probably shorten this phase as they are altready at ceiling levels of fitness.