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When you don't have any energy left to keep pedalling. We call it bonking. Its quite alright. Everyone does it at one time or another. You are being sensible to walk up the hill instead of struggling.
And I think you are doing just great to be out and riding. NO ONE EVER GOES OUT THE FIRST TIME AND AVERAGE OVER 20MPH for 100 mile stretch. I returned to cycling after 6 year hiatus after a major crash. The first time I went out last year, my butt was numb after only 10 miles and it took me an hour to ride.
Shawn