Mile races and marathons are different ball games.
Like another response, you have great speed but lack strength. It takes a long time to develop aerobic endurance.
How old are you? I have high school athletes who can go from off the couch to about a 5 minute mile in 2-3 months. But they wouldn't be able to go sub 1:30 in half because they don't run in the off season.
If the 1:35 pace was a hard effort, I don't think you could do that for a full marathon in such a short amount of time. It's a great long term goal.
Also another great question already asked is how much have you been running per week this last few months? Don't increase too soon.
You seem ambition, which is wonderful. Running requires patients and discipline. Just be smart and take a long term look at where you want to go.