Run: Active "Rest" on the Rowing Machine & Equivalent Cross-Training Mileage

Today I Erg'ed.

I'm trying to give my poor legs a break from the beating they're taking from all this running, so I hit the rowing machine today to get 30 minutes of cardio in.

I thought about trying to record the approximate equivalent mileage as running mileage, but I think I'd rather just consider this a "rest" day. Recording approximate equivalent mileage isn't really all that helpful, as far as I can tell, because the mileage one records in a running journal is to help determine overall fitness to run, which isn't just about cardio fitness, but also about your body's ability to take the specific physical stress on muscles and joints that running creates. So you couldn't try to do half of your mileage cycling and calculate the running equivalent and expect to run as well as someone who just runs all those miles. Cardio-wise, you might be as well-off, and you could probably bike circles around the runner, but your legs just wouldn't hold up to the punishment of running without actually running enough training miles. Just ask Lance Armstrong how he felt after running the NYC Marathon, and you'll see what I mean. Granted, Lance wasn't in the best shape of his life for the marathon, but this is a guy whose cardio-respiratory fitness will probably always be up there amongst the top 1-2% of people in the world no matter what.

Maybe if you're specifically cross-training as a strategy to save your joints, muscles and bones the wear and tear (or to alleviate the tediousness) of running it might make sense to try to figure out the equivalent mileage of various activities, but it's always going to be crap-shoot.

Anyway, after some research, here's some basic formulas for calculating equivalent running mileage for different activities. I can't cite any sources for any of this, so, um, you know, don't blame me if you follow these formulas and your training gets all funky and stuff.

Swimming
1 mile (pool) = 4 miles run
1 mile (sea) = 6 miles run

Cycling
4 miles = 1 mile run
(some would say 3 miles biked = 1 mile run... who knows?)

Rowing
2 miles = 1 mile run

Elliptical
I couldn't find anything on the elliptical, except that many runners swear by it because it more closely simulates the motion of running without the impact, while others question whether it's a good substitute for running because in some models (notably the Precor)the weighted flywheel that the pedals are connected to actually contributes a lot to the momentum of the movement, so you may not be getting as good a workout as the machine indicates. Ultimately, if you're using your heart rate to determine effectiveness of the exercise, then it doesn't matter. But if you're tracking calories burned, don't trust the machine. Actually, come to think of it, most machines are probably not very good at tracking calories burned.

A General Rule
If you're checking your heart rate, you can approximate your equivalent running mileage by taking your average heart rate during the exercise, figuring out what your normal running pace would be with that many miles run, and then divide the total time you exercised by that pace. So, if my heart rate is 170 for the 30 minutes I erg'ed today, and I normally run a 9-minute mile when my heart rate is at 170, then my equivalent miles run would be 30 divided by 9, or 3.33 miles. (This general rule courtesy of Bob Glover, one of the authors of the classic The Runner's Handbook.)