Originally Posted by
i wish
I've been riding about 3 days a week. Supposedly if one gets one"s heart rate up for about 20 min 3 or 4 days a week that will get one into shape....
Anyway...I do a 10 min warm-up and cool down. In between I ride hard 1 min, and light 2 min for about 20 minutes.
Also, I ride once a week to a place ~3 miles away and then ride back a few hours later. I don't try to ride hard, because I don't want to get there sweaty...but it is a little bit of time on the bike....

I don't have a huge amount of available time...preferably I don't want the workout to last more than 45min to 1hr.
No ... doing such a little amount of exercise might help you
maintain your fitness level if you were going through a few weeks or a month or so when you couldn't exercise much, but it won't help you get into shape.
The only way that will help you get into shape is if you just got off the couch. That amount of exercise would be good for your first 2-3 week ... and then you'd want to step it up a bit.
After doing that for a couple weeks, aim for at least an hour a day 4-6 days a week of moderately intense exercise for the next few weeks to build a bit of a base. As I suggested earlier, if you can, join a 1-hour spinning class twice a week, and then do the rest at home.
After you've done that for a few weeks, then pick one day a week, and start increasing your distance. Presumably in your part of the world, that will be just in time for spring. Get out for a 1.5 hour ride one of those 4-6 days. If you felt comfortable with that, try a 1.75 hour ride the following week, and gradually build up on that one day a week.
After doing that for a few week, add in a second day of longer rides so that you're doing back-to-back longer rides. This will not only help you build your fitness, but will help you get used to being able to get up again the next day and ride.
You are 21 years old ... you should be in great shape!! Get out there and get into shape!