This all sounds a bit like when someone gets voted off the X Factor on the first week and declares "this won't be the last you hear of me" when you know full well it is.
I also wasn't trying to be snobbish. Just stating that as a runner I find it odd that some non runners think they know it all. I wouldn't dare of going into a gym acting all cock of the north and telling everyone how they should be doing things.
There are probably plenty of Olympic standard athletes that are dumb. They just follow what their coach says and don't think for themselves. They are lucky that they have a coach though.
I'll never forget the morning a bunch of "gym types" turned up at parkrun with cross fit t shirts and tough mudder headbands. All pushed right to the front but were left for dead by a group of middle aged skinny men.
If you had a few marathons under your belt and were used to bashing out 20 milers then you may be able to aim for 3.10- 3.15.
As you don't have any experience I'd aim for 3.20 -3-30 and then move on once you have a marathon training programme under your belt.