It's hard to top running for an aerobic workout. You simply put on a pair of shoes, enough clothing to meet the environment (or common decency, if you live here in the tropics) and off you go. The thigh muscles are the largest muscles in the body, and you get more metabolic impact running than from swimming, rowing or other exercises that work the upper body. (Disclaimer: a kid I work with was on a college crew team: his arms were as big as my legs). You shouldn't ignore your upper body, though, because you may need to carry things around, perhaps chop firewood, or do other things that require upper body strength.
But what about people who can't run for medical reasons?
If you have medical issues that prevent running, you're not going to be on the front lines unless something terrible and unanticipated happens; you're going to be holed up in your home or retreat trying to avoid contact with the outside world. There are many people that simply should not run - even if caught in a downpour, or another situation in life where most of us would. They may be only able to run just far enough to get out of the way of an oncoming car and may require surgery afterward. If you've had a hip or knee replaced, your orthopedic surgeon has already told you this. If you're “working toward” getting that replacement, with arthritic hips or knees and already have reduced mobility, you already know you can't run. There are also people who may be able to run once in a while in an emergency, but who would be hurt by the steady training.
So what can you do? Anyone can walk. Dead people walk; you can see them mall-walking in groups every Sunday morning around most city malls. Or you can ride a bicycle.
Western Rifle Shooters Association linked to this article on a “Couch to 5k” running program that started me thinking about this. If it doesn't immediately register, 5k is 3.1 miles; it's also about the most you should consider running regularly, unless you're young and training for specific running events (according to the M.D. who wrote that “Cardio May Kill You” link). It's hard to equate running miles to cycling miles, because the difficulty in cycling goes up more than that of running when the road tilts up, and approaches zero on a downhill, but on level ground a 3.1 mile run would be equivalent to cycling about 12 miles. As a runner, I've never run further than that, while as a cyclist, I've ridden across Florida (120 miles), a testimony to how low-impact cycling is. In running, the marathon (26.2 miles) is considered the benchmark endurance event; in cycling the equivalent is the century (100 miles) and you can use that roughly 4:1 ratio in miles. Another way to equate the effort is that 5k running plan aims at getting you to 10 minute miles, making the 3.1 mile event take 31 minutes, so your cycling equivalent would be to cycle at moderate intensity for that amount of time.
Taking that running plan and translating it to cycling is fall-off-a-log easy. Where it talks about running hard for 60 seconds and resting for 90, for example, just substitute riding hard vs. riding easy. I'm a fan of cadence meters on a bike (part of your cyclocomputer), but I know that adds expense. If you have one, you can consider cadence in your workouts, warming up by spinning at a known rate (90 pedal rotations per minute, say), and then working to maintain that cadence when you're riding harder. A high cadence minimizes stresses on arthritic joints. Yes, you can measure cadence by counting your pedal revolutions in 6 seconds and multiplying by 10, but that requires coordination. It's easy to read a number off your bike computer.
Compared to running, road cycling is extremely low impact. In the days after knee or leg surgery, often the first thing that your physical therapist will have you do is ride a stationary bike. On a good road bike, on flat roads, you can do the same level of work on a freshly post-surgical leg; I was on a stationary bike 4 days after arthroscopic knee surgery and my own road bike 9 days after the 'scope.
The problem, of course, is that cycling requires a bike. While a runner can get a good, if not top end, pair of shoes for around $100, a good entry-level bike is going to cost 4 to 5 times that. While the department store bikes may be lower priced, they usually have features that make them a poorer choice like steel wheels as opposed to aluminum alloy wheels on “bike shop” bikes (the issue isn't the weight; it's that the cheaper chromed steel wheels are harder to stop – especially when wet). There are decent bikes in some sporting goods stores like the Sports Authority; I've seen Fuji at my local store (a few years ago), and Diamondback bikes. In general though, the bikes at a bike shop will be a better choice for a handful of reasons.
One reason is that a bike shop will help you get the fit right; and the fit is vitally important (I can't emphasize that enough). A friend I was helping to buy a bike was surprised by my talk about bike size. He remembered riding a 26” bike as his last one as a kid, and just assumed that's all there was to know. 26" was the wheel size, and bikes with the same size wheels come in a wide variety of frame sizes, and different geometries, which vary the amount of reach to the handlebars. It drastically effects how comfortable the bike is to ride.
Most bike manufacturers today offer a bike specifically for the fitness rider, or someone who has not been on a bike as an adult. Called a city bike, an urban bike, a fitness bike, a hybrid, or sometimes a cruiser, they have a more relaxed and comfortable geometry than you probably are thinking. The geometry determines the position you sit while riding, from the aggressive, head down position of the road racer (not recommended for a beginner!) up to a rather high sitting position, excellent for seeing around in traffic. Some will use touring bike tires (metric 700C size) while others will use mountain bike tires (American 26” size). A selection of these bikes might be hard to find in a small shop, but the manufacturers all offer them. Incidentally, high end bikes are still an industry where America is very strong; more Tour de France winners have ridden American bikes than European in the last 20 years. While they all have offshore factories for their low priced bikes (typically in Taiwan, not the PRC), the high end Trek, Cannondale, Specialized, or Cervelo bikes are made in the US.
(Trek's popular 7000 - a hybrid that retails around $425)
In addition to a bike, you'll need a helmet. I find padded cycling gloves to be essential - and they'll help protect your hands when you fall. Add a water bottle, and a cycling computer (speedometer) and you'll easily be close to $100 over the price of the bike. You don't need clip in pedals (which require special shoes) or even the old-school toe clips. For short rides of 35 or 40 minutes, you won't need the dreaded, padded Lycra shorts like this model is wearing, either. Any old gym shorts will work.
(upright exercise bike that retails for about $250)
Indoor, stationary bikes are certainly an option and may be best for some people and places. Personally, I hate 'em. If I'm outside on a pleasant day, it can be tough to make myself go home. Indoors, I've never wanted to ride an instant past the timer telling me I'm done. Guess which one is better for overall fitness? Still, there's a nationwide trend called Spinning, done entirely indoors on stationary bikes, and it's a good workout. And if you're not an experienced rider, I think riding outdoors on snow would not be good way to start.
In the case of both regular bikes and stationary bikes, used ones are frequently available. Since stationary bikes are more adjustable, finding one isn't as hard a finding your exact size in a bike. Plus, exercise equipment frequently becomes an extra clothes rack while it's still new, so they might be easier to find.
If you'd like to discuss types, brands, models or anything - we can do that, but my main point was to remind readers that just because you may not be able to run, that doesn't mean you can't get a good cardio workout and be better prepared for whatever may be coming our way.