Just ran a 1:45 HM. Time prediction for my first full marathon in 6 weeks?

First full marathon in 6 weeks which will be a flat course. Yesterday I ran a 1:45 HM that was net downhill but still fairly hilly. 8 min HM PB (this was my fifth half I’ve ran).

Main goal for my full is to finish and secondary goal was sub 4. My fitness has increased over the past year since I got back into running (started running 8 years ago with a couple years hiatus) so thinking of adjusting my time goal. 3:45 too much of a stretch?