When I cycle, I assume that I've burnt about 500 calories per hour. So if your 21 miles took you about 1.5 hours (like it would me), that would be about 750 calories.
I also assume I burn about 1500 calories just sitting around doing nothing, based on my height and body frame. I'm a bit shorter than you, so you might want to assume 1600 calories or so. Look up basal metabolic rate on Google for some calculators. Try different ones because they'll all give you a different number. I pick a number at the lower end of the selection.
So for me, that would be a total of about 2250 calories.
Now if you're trying to gain weight, which I assume you are because you are quite underweight for your height at 120 lbs, then 2800 calories should have you gaining a pound or so a week.