I've made some changes to my marathon plan, thanks to comments from Randy, one of the advanced runners on the about.com running forum. I decided that my present mileage is more like the new week 6 rather than the old week 4. So, I planned to do 5 today, but I started feeling a bit tired and turned around for a distance of 3 miles -- I didn't want to overdo it and mess up my 8 tomorrow. I didn't get enough sleep this week, and that contributed to my tiredness this morning (had a great sleep last night, though). My wakeup HR was 52 this morning, so my tiredness isn't great.
My new marathon training plan has the runner running 6 days per week instead of 5. The runs are one long run that advances up to 15 and then to 20, one medium run that stays at 8, and four short runs, two 4 and two 5 milers. It's the same weekly distance as the old plan, but the miles are spread over an extra day, and there is one less medium run and two more light runs. The result being less stress for the same weekly mileage. Until I retire in June, I'll only be running the Monday, Wednesday, Friday, and Saturday runs. That gives me one long run that should be up to 15 by the end of April, one medium run of 8, and two short runs of 4 and 5 miles.