First of all, since you're only doing 10mph ground speed you've probably shifted down so were using a lower gear and therefore had enough torque. In many ways it's comparable to climbing a long shallow grade. You might also have made it easier on yourself by assuming a lower profile than normal to cut wind drag. So while you were pedaling hard enough to go 30mph (wind speed) you're only covering 1/3rd the distance.

The reverse happens on the downwind leg, at 25mph you only had an effective wind speed of 5mph, allowing you to fly along in a much higher gear than you could sustain in still air.

This effect is the main reason that I set my bikes up so my "cruising" gear is 2 down from high. That leaves me roughly 20% "overdrive" for downgrades and tailwinds.