I incorporate weights into my training but always struggled in the latter part of marathon training to fit it in. Now only do 2 weight sessions per week, medium weights and high reps (15-20 per set). Overall it does help especially recovering from injury.
troublesome, I know where you're coming from. After training over hilly terrain for weeks at above MP I was sailing well for my PB of 3.15 only to cramp up at 17 and then fall apart to come in at 3.40. The heat was brutal and I for one couldn't get any more gels, water or electrolyte solution to stave off cramp or dehydration. Don't be to hard on yourself as there were some poor souls who were in a right state and I'm thankful I finished in one piece.