Clinton Lake vs Road Marathon

Last year, a friend and I hypothesized that two 10-mile loops of the hilly Clinton Lake trail would equal the time it takes to finish a fairly flat road marathon. It seemed right. It felt right. Well, I finally ran a road marathon (Illinois Marathon in 3:47). So how does that 3:47 compare to a recent two loop (20 mile) run on the Clinton Lake trail?  My only recent 20 miler at Clinton Lake was part of the 30-mile race there in March. I did 3:40 for the first two loops (20 miles). So my 20 miles at Clinton was either too fast, or my marathon too slow. Or this predictor doesn't work!  Actually, I'm impressed it's that close. The Clinton race was a RACE, not a training run. If I was training, it would have been just a bit slower. Maybe 3:45. And my road marathon was "raced" very easily. I could have run faster. Maybe 3:40. See the error variance in these measures? I need more data, but it may work.

So, I think the "rule of thumb" that two loops of Clinton Lake equals a road marathon is about right. That's my story and I'm sticking to it! I'll complete several 20 milers at Clinton Lake in the next few months so I'll have a measure of my fitness and a possible predictor of future road marathon times. Nice. Let's get that 20 miler down to 3:30 and I'll be ready for a Boston qualifier.

Another interesting side note: I finished 30 miles at the Clinton Lake ultra in 5:42 and finished 26.2 miles at the Illinois Marathon in 3:47. Both were raced "under control." Wow, an extra 3.8 miles at Clinton Lake took me an extra 1 hour and 55 minutes! That Clinton Lake trail is tough.