Hiya, this'll be my 15th so I know a little about how my body responds
3 months ago I wasn't training, so I was struggling to knock out 11:30 a mile over distance, i'm doing the Luton marathon in one week today, and i'm going for 4 hrs since training has gone really well and i've lost 24lbs (not such a lard arse now!). If I get somewhere around my target and don't crash and burn, i'll probably set my sights at around 3:40 as my pb is 3:41, it would be a nice birthday present as i'm 40, 6 days after Rome.
Your time will probably depend on how many long runs you do, and how many you've done before, you're 10k time would get you sub 4 easilly, but only if you train well enough for the marathon.