There's no right answer to which one is better; they both will help you get in shape with almost no injury risk because they're zero-impact (unlike the pounding you get from running). You should choose the exercise that's most convenient for you and that you enjoy the most.
Start with an extra workout of 30 to 60 minutes per week. You can add increments of 10 to 15 minutes every week, or another session every two weeks. Here are a few key pointers to remember to ensure proper form and execution:
- When pool running, keep your back tall, ensure a high cadence of at least 90 leg rotations per minute (for each leg), and don't overextend your legs.
- For better form while pool running, use a flotation belt so you don't slouch.
- If you're cycling outside or inside, keep your cadence to at least 90 per leg. This will help mimic an efficient stride.
Estimate the amount of supplemental exercise you're doing by using this approximate conversion formula: 15 minutes on the bike or in the pool is the equivalent of running one mile. You can build your fitness by adding extra "miles" to your weekly training with none of the impact stress.
Of course, as you become a more experienced runner, you'll need to run more mileage gradually and complete more difficult workouts in a progressive manner. There's no shortcut around hard work—especially if you have an ambitious goal such as qualifying for the Boston Marathon.
But supplemental exercise can act as a bridge between all the running that you want to do right now and what you can reasonably expect from your body. Keep your effort level at a comfortable or moderate intensity and you'll gain more fitness than you would with just running. And your race times will prove it!race.