Next few weeks looks good Roger, well done on 15 on Sunday. I personally don't drink on any training runs under 16 miles just take 1 or 2 gels. SIS seem to work well for me as you don't need water with them. As for marathon day would maybe take on 5 gels at 6/10/13/17/21miles. 1 every 20 minutes sounds to many to me but we must do what works for us.
8 at 8m/m then 8 at marathon pace sounds like a testing run, enjoy.