To be honest, if you are worrying about the distance with only 4 weeks to go then you have left it too late.
There are a number of factors to think about before a any descent advice could be given.
- Current fitness level
- Type of terrain that you will be riding.
- Amount of time available to train before hand.
Really this leaves you with 3 weeks to train so that you have a week (or the best part of a week) to recover fully for the ride.
If you can only get out for 3 or so times a week then I would say try and push the distance you can manage a bit (not too much, you do not want to burn out). Keep it consistent and don't increase the amount you do in a week by more than 10%.
If you can do 5-6 days I would suggest short training sessions with some interval training and one longer ride a week.
Recovery is just (if not more) important than the exercise, (but have to exercise to recover!). If you push too hard or do too much you will take too long to recover and not gain much (or could loss) some fitness by the time of your ride.
The problem is with only 4 (3 really) weeks left to your ride it is hard. If you do not know your body and your limits now it is possible you will either under train (to avoid burning out), or over train and burn out leaving the long weekend a dream, or a very painful event.
Hope this helps a bit, and good luck.