A few years back I went on a fitness drive and managed to go from 11 stone down to 8.5 stone in 6 months through a combination of running, cycling, skipping, walking and eating 1600 calories a day. At the time I measured everything dogmatically. I discovered that to walk at 4mph for any distance is actually quite difficult for me and feels like a work-out. At my usual dog-walking pace in mixed terrain a mile takes me roughly 17-18 minutes. I was always cycling as a kid and teenager and thought nothing of cycling 20-30 miles first thing on a weekend getting home in time for my mum making family breakfast. By contrast, although quite a good sprinter, I really struggled with running a mile as a teenager and years later discovered this was all down to poor technique and lack of faith in my own ability. I learned to swim in the sea as a child and can swim miles without feeling tired - although I do get hungry. Now I find it quite difficult to cycle at more than 12-14 mph (although this is improving) but I can run faster than I've ever run before at the age of 45 and I can still swim a mile without much effort at all and in less than 40 minutes. So I take the comparison statistics with a pinch of salt as I think it depends a lot on the individual. It's still interesting to see what people think though i.e. a marathon being the equivalent of 140 miles on a bike. That sounds plausible.
