How cautious should i be with midweek miles?

training for the Yorkshire Marathon from Leeds and i'm trying not to turn every run into a mini time trial. i did my first half last spring and felt fine on 4 runs a week, but marathon training is already feeling like a different thing. My plan has me around 32 miles this week, with a 16 mile long run on Sunday. Does anyone else find the Tuesday/Wednesday runs sneak up on them more than the long run? Legs feel okay, just heavy. Wondering if i should slow those right down or trim a couple of miles before it becomes a problem.