Starting corral

Running Rome marathon on Sunday. Have been training for a finish time of 3hr or less, and I'm reasonably confident I'll be able to achieve that according to how my training has gone. However, as this is my first official marathon I have been placed (according to me bib no./colour) in the 4hr starting corral which has 3hr40mins as the fastest pacer.

I've not run a marathon before so I'm wondering if I start at the front of this group will I be able to run at my target pace without much difficulty, or will I be hampered significantly by having to weave around slower runners at the start? I'm disappointed I won't be able to start with and stick with the 3hr pacer as mentally I think this would have been an easier task.