Measuring Progress by Effort

If you’re cross-training because you’re too injured to run, then you want to have some general idea of whether you’re doing enough in other sports to compensate for your lack of running. Even when you’re cross-training to supplement your running, it’s nice to have some general idea of how much work you’re doing in a language you’re used to. So at the risk of contradicting what I said in the preceding paragraph, try this: Figure that you’re getting roughly the same cardiovascular benefits as running for every minute that you cross-train at a similar intensity to the level of effort that you normally sustain while running.
If 4 miles in 40 minutes is an average, medium-effort run for you, then sustaining that level of effort for 40 minutes on a stair machine will provide roughly the same aerobic benefits. Your heart doesn’t care what’s getting it going, so long as it’s an activity that uses major muscle groups for an extended period. Admittedly, this system is imprecise, but I would rather add cross-training to horseshoes and hand grenades and consider “close enough” as being worthwhile than worry too much about exact trade-offs.
After all, those same runners who so precisely say, “I ran 43 miles last week” are deluding themselves unless all of their running is on precisely calibrated courses. Most days, they’re making rough estimations about how far they ran based on the amount of time they were out for. I don’t see much harm in taking the same approach to cross-training. However, notice that I said, “every minute that you cross-train at a similar intensity to the level of effort that you normally sustain while running.” That’s different than just counting the number of minutes that you cross-train. On a stair machine, for example, you’ll often spend the first 10 minutes or so getting going; if so, those 10 minutes aren’t really at the same intensity as the first 10 minutes of most runs. During a two-hour bike ride, you’re likely to spend a nice chunk of that time going downhill, drafting, coasting, and so on.
If you’re new to a sport, it can be difficult to gauge accurately how hard you’re working. It might seem as though you’re really putting in a good effort, but that could be because your legs are easily tired by the unaccustomed motion, rather than because you’re working at a high heart rate. Many runners experience this kind of fatigue on their first few bike rides.
If you have a heart rate monitor, use it when you cross-train to gauge whether your pulse stays in the neighborhood that it usually does when you run. Otherwise, expect to spend at least the first few workouts in a new sport learning how to assess your effort.