Your HM time is good enough to put you in position for sub-3. I went sub-3 in a debut marathon off a 1:24 HM. But as you probably know, your mileage is not sufficient to pull that off unless you're particularly talented.
I think the low mileage makes predicting a crapshoot. I would also put the over/under around 3:15.
Not sure if you want advice, but since you have 2.5 hours for a long run on the weekend and limited time during the week, I would consider doing the following:
1) Make the long run count. Rather than just time on your feet, start adding goal marathon pace work and possibly tempo pace rather than holding the same pace throughout. Teach yourself to run goal pace on tired legs. Since you have more time to recover than others given your low mileage, this shouldn't hurt you.
2) Increase the pace of your tempo run. Shorten the distance if you need an adjustment period. A 1:25 HMer should be able to run at least 6:30 pace for tempo. In theory, 6:50-7:00 is more like marathon pace - if you prefer to practice that midweek, I'd extend those MP runs up to 10 miles, but you may not have the time.
3) Add a speed session midweek. I personally think tempo and long run are good enough for a marathon, but if I were working with your time and mileage limitations, I'd want to make every training second count. So I'd do something like speed Tuesday, tempo Thursday, long run on the weekend.