Zigzag - aren't you a significanty stronger cyclist than runner? Isn't that what it's about?
How Many Calories Do You Burn? - Weight Loss - Runner's World
Using those numbers, it looks like a 4hr marathon would give me similar numbers to those I got off Sporttracks for a 13.5hr 300k. However, I know that Physical Activity Energy Expenditure (It's an activity measure used in predicting obesity - I am sat in an MRC Epidemiology dept today, so have be talking to one of their researchers who runs marathons about this) weights intensity from HR when deriving energy expenditure from accelerometry, so I suspect to get a metric that can compare across disciplines, you'd want something that mixes energy expenditure with intensity, so you can compare across similarly trained people. The guy I was speaking to said that they didn't use any measures like that that, but they'd be easy to derive, albeit arbitrary. Simplistically, something like kcal + kcals burnt above lactate threshold (so those above lt count twice) is what I am thinking of.
Still, I can't imagine many people are equally trained /suited for multiple activities, so not sure what use it'd be.