Which marathon are you running? I ran the cincinnati one last year and have done 2 half marathon's so far. My goal was just to finish the race and I did in 4 hrs and 40 minutes. I basically tried to maintain a 10 minute mile pace. It is a very physically and mentally draining experience and takes a lot of training to accomplish. You will find some days when you fell really good when you run and others when you dont.
My recomendations are as follows
Go to a running store and buy a pair of shoes. They will fit your foot and it will help greatly. You can alternate shoes but I just had one pair.
Buy a few good pairs of synthetic socks (smartwool), running boxers, and a thing called body glide.
Hydrate. I gave up drinking alcohol about a month before the race and quit drinking coffee and pop 2 weeks before the race.
I lift weights very frequently in the winter and not so much in the spring because I golf quite a bit. I would say that it is ok as long you do lighter excercises focusing on your legs and core you will be ok. I rarely did any chest or bicepts during my training.
3.5 hours is a pretty good pace. If you were a former athlete you probably could do it and it is a good starting point. I would not be opposed to move the goal higher if you feel that you can not reach you pace. You will get a better felling of how fast you will be able to run when you start doing your longer runs.
I would get a watch or anther runner to set a pace with. Pace is very important because if you take something out too fast you will not have enough energy to finish.