Hi markymark
40 Hours !!!!!!! How big is your hard drive !!!
Use the pseudorandom single pass, it will be secure enough for most if not all purposes and I suspect it will be much quicker than the current method you are using.
You cannot delete in stages. If you stop the free space wipe and then use your computer you may write and then delete to a part of the disk that you had previously overwritten.
Free space wiping has to be done in a single event.