Difference between 70% N1 and take off power can be very small in some cases, we use N1s as low as 75%. With todays derate/assumed temperature stuff take off power can be very low indeed, especially on a long runway.
The brakes should be able to hold the aircraft at 70% N1, however on a slippery runway in a light aircraft that might not be the case in which case one does not do the whole 30 second runup, just up to 70%, check if engine indications are ok and off we go.