In my PR years, during peak summer season, I was logging 40-60 running miles a week, 200 miles on a bicycle, and just enough swimming to keep from drowning in an Ironman.

Health & Injuries?

I have no idea. I once heard that a mile of swimming is about equivalent to 4 miles of running -and in terms of the amount of time required, that doesn't seem wildly off the mark. Pfitzinger suggests that a minute on a bike is about like 20 seconds of running, which also makes sense: a bike century ride (100 miles) takes five to eight hours, and feels (if you're equally fit in both sports) about like a 20-mile run. But it's not linear: a double century (200 miles) is a lot less demanding (once your posterior recovers) than a marathon, let alone a 40-miler. And skiing, I've managed three long days (60, 40, and 60 kilometers) back-to-back-to- back, on not a vast amount of training, all at race pace. I can't imagine trying anything remotely like that running.

The truth of the matter is that it's apples and oranges, and the low-impact aspect of cross-training makes it a lot easier on the body than running -which is, of course, the whole point.

A related question is how much to do. If you're an adventure type like me, you're going to want to do more cross-training than you need, if your goal is solely to improve your running. And if you run a lot, you're going to need less cross-training to supplement your training.

Liljeblad suggests that if you take up bicycling or swimming, you might try doing them twice a week -though if you're doing both, you might want to hold the total to only three. On the other hand, bike commuters get in 10 cross-training workouts a week (two each day). Ultimately, it comes down to what you want to do and what your schedule and body can accommodate. The main thing to remember is that cross-training, for runners, is recovery. If you start hammering, you're going to have to reduce your running to compensate.