Marcus Grimm is one of five Boston Marathon entrants testing the Polar RCX5 personal training computer and blogging about it for Boston.com

By Marcus Grimm, Boston.com Correspondent

I’ve often thought that if you really want to see what a person’s marathon plan is made up of, you could just skip ahead to the last long run, exactly three weekends before the big race. In that one workout, you can get a feel for how difficult the entire plan has been.

In many plans, my last long run consisted of around 20 miles, with 10-12 at goal marathon pace. In most cases, I found that workout difficult (or impossible) to complete. But this time around, I hit that mark quite early in training, which meant my coach had to come up with a new variety of torture for my last long run.

She did. After a mostly leisurely four-mile effort, I dropped into a serious of five 2 mile intervals. The first half mile was at 6:30 pace, followed by a mile at 7:15 (approximately my goal marathon pace) and a half mile at 8:00 pace.

The first four intervals were easier than I’d expected, but the fifth was tough. Real tough. And after that, I still had six miles left on the plan. Four of them were easy, but that 19th mile called for a 7:15 effort. It was tough, but I was thrilled to pop a surprising 6:52.

Plotted on my Polar Personal Training account, you could pretty much read the “tale of the tape,” by looking at my heart rate. The good news, I felt, was that even by the end of the workout, the marathon pace efforts weren’t too taxing from a heart rate perspective.

Of course, heart-rate and pace weren’t the only number I was watching. As a type 1 diabetic, the last long run is also a “dress rehearsal” for the marathon. I’m matching my gel intake to my insulin to ensure an ideal race performance with a blood sugar between 80 and 120 during the race. There, I did well, too, starting the workout at 85 and finishing at 110.

So with that begins the three-week taper process, a period of hoping what you did was enough to get what you can come Patriots Day. When I’m in doubt, this will likely be the workout I pull up for affirmation that what I want is within my grasp.