I just finished my 30 miles of biking and one thing I forgot to mention is that when you get to the end of your workout--the hard part where it's hard to breath and your lungs feel ,like they're on fire--that's the time to push even harder.
I try to up my speed several miles an hour for my last five miles when I'm feeling like I'm going to fall off the bike if I have to go one more mile.
It's a great way to get your body conditioned and to burn even more calories.
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ETA--of course I agree about being knowlegable about what one is doing.
However I have seen too many people get discouraged by all the numbers and calculations and just give up on their workouts when it is really more important to do the activity than to worry about decimal points on a chart.
IMO based on my own experiences and from watching others getting started on a program of improving their health.