It's your first half. Don't worry about speed. By my calcs even if you did 13 on the Sunday you are doing 19miles slow/easy and 12 miles speed. Really speedwork should only make up 20% max of your weekly mileage so that's about 4/20miles. Swap the tempo for a slow run of maybe 8miles. And alternate weeks of tempo/speed work.
I think you have to be careful how you approach the half marathon during the lead up to the marathon.
I've done it two ways. The first I think was the best which was to run the half at marathon pace to just practice race day prep and see how I felt at the end of 13miles. This involved no reduction of mileage in the week before and didn't affect training the week after.
The second was an all out assult on my half marathon PB which resulted in a 1:48 but meant a week taper and two week recovery afterward and I don't really think I recovered for the rest of my training and ultimately the marathon itself went very badly.
That's just my 2p worth. There were also undoubtedly other reasons for my bad result.
The research I read with gels about the instant boost was that sensors in your mouth trigger the brain into knowing sugar is on its way. Anything sweet will do. Polos, Jelly Babies. But I've found I'm running at such a pace anything I swallow gives me stomach ache more or less straight away.