“I missed my long run (12 miles) yesterday and ended up biking 30 miles instead. I know that when it comes time to training muscles, there is a big difference between biking and running, but all is not lost, right? I tried to push myself on the bicycle, so I was getting a good workout.”
No, all is not lost. I have seen charts comparing run workouts with bike workouts, the most famous being those in Dr. Ken Cooper’s best-selling Aerobics books, but I don’t put too much faith in the numbers. The main reason is not the runner, but rather the bike used by the runner. A skinny-tired bike might be twice as fast as a fat-tired bike.
A muscular cyclist might have an easier time pushing up the hills than a skinny runner, who in turn might be faster on the flat. So was your 30-mile ride better than, or at least equal to, your missed 12-mile run? I can’t offer even an educated guess. Don’t take too seriously anyone who claims they have the right answer.
Consider also that training is sports-specific. To become a better runner, you need to focus most (if not all) of your attention on running. Similarly, to become a better cyclist, you need to spend most of your time on the bike. That does not mean you should never cross train; just be aware of what your main sport is when choosing workouts.
One other comment: Rather than pushing yourself on the bike to get a good workout, you might have been better off sitting back in the saddle and cruising. If scheduled to run those 12 miles at an easy pace and you chose a hard pace for your 30-mile ride, you might have done more harm than good by developing the wrong muscles. Don’t convert an easy workout into a hard workout, simply because you can.