wozer 83 - the usual advice is to increase by no more than 10% per week, in order to avoid injury and give your body the opportunity to adapt and repair.
when you say you are up to 17 miles already do you mean per week or for your long run?
Most marathod schedules are around 16-18 weeks and won't have you doing 17 miles until around weeks 6-8 so I would think with this in mind you might be advised to back off a bit and start a steady build up hopefully peaking on the day!