I ran a marathon in 2004 and don't classify myself as a runner either. My goal was 4 hours and I ended up doing it in 4 1/2 because I bonked with about 4 miles to go.
I was in really good shape when I ran it and I'm about the same size as you. If you aren't a runner 3 1/2 is a pretty fast pace. Its definately possible but for your first marathon might be lofty. In my training my longest run was 22 miles and that's where I died in the actual race.
With your question about lifting I would say stop. You really don't need to lift legs at all. I actually found that my legs got much stronger from running than they ever did when I lifted.
Best advice I can give you is to get a good pair of running shoes. When I started training I was just using cross trainers that I always wore and after about 12 miles my feet would kill me. Naturally I thought that was just because running 12 miles is no small task. Well about 6 weeks before the race I went to a running store and had someone get me a good pair of shoes. My feet never hurt again.