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

As the hours came closer to the start of the Boston Marathon in Hopkinton, it became painfully clear that us runners were facing practically unheard of situations in regard to the weather. The conversations among all the runners weren’t about if we were going to slow down, but how much we were going to slow down to avoid the dreaded DNF. Trust me: after you’ve bought a lot of “Boston Marathon” gear, there’s extra incentive to cross that finish line!

In my case, I’d decided to slow my marathon pace by 15 seconds per mile, but I realized around Framingham that doing that resulted in a heart rate of around 85 percent of my max. From experimenting with the Polar, I’d come to know that I can do a long, hard workout at 80 percent of max. But 85 percent, to me, seemed dangerous, so I immediately slowed my pace another 5 seconds per mile, and dropped my heart rate closer to the 80 percent number.

I’d love to tell you that this simple change turned my 2012 Boston Marathon into a wonderful experience, but that would be a lie. It was the hottest marathon, by far, that I’ve done, and I spent much of my time hunting down the “angels” along the sides of streets with bags of ice. They became my heroes for the day! But that data from the Polar pulled me far enough from the brink of disaster to keep on keepin’ on!

In the end, I finished in 3:33, very close to my slowest marathon ever, and a full 15-20 minutes slower than I expected to be. That said, I finished in around 3,500th place. Considering I was bib No. 8348, in the end I handled the heat better than a lot of people. And for that, I give all of the credit to the Polar. If you visit my splits and heartrate from the race (which I started a minute prior to crossing the start line), you can see I averaged about 83 percent max for the race, which shows that given the conditions, I gave it my all.

As runners, we often spend too much time focusing on our times and not enough on our experiences. I’d like to thank the Boston Marathon for providing me with my most incredible marathon experience to date. When I think about qualifying for Boston a year ago, I think about how proud I was of my result. When I think about the Boston Marathon of 2012, however, I think about a lifetime of memories I’m grateful to have.

Thanks again to Boston.com and Polar for the opportunity to share my journey with you. Please feel free to visit my blog.