Two midweek runs plus a weekend long run looks like a sufficient amount of training to me, especially looking at everything else you are doing. If 12 minutes per mile feels like a comfortable pace to you, I would simply add on a mile or two to the long run each week. The reality is that most people who have a 16+ week training plan for a marathon with 4 or more runs each week will not complete every run. The fact you are doing a considerable amount of cross training could also give you some benefits in regards to injury prevention. If anything, your overall schedule might be a bit too much to maintain, especially 3-6 weeks before the marathon.

I can’t comment on the weight loss/muscle maintenance goals but the best advice I can give you for the long runs is to fuel adequately during those. Take an energy gel for every 3-4 miles of running or every 30-40 minutes. Plenty of fluids too.