Does 30 days make a habit?

The 14th will be a month since I started seriously, on a planned schedule, exercising. I thought I'd feel better by now. I'm more tired now than when I started. I think perhaps 60 minutes a day is too much. It is a 20 minute jump from last week. My legs are killing me and my energy levels are down.

I might just stick to 40 for now and up five minutes a week until I reach 60 again. But next time more comfortably. I don't want to get so tired and discouraged I quit.

This is however, the longest I've kept to a schedule. Is it 30 or 90 days that makes a habit? The way I feel at the moment, it must be 90.