Cycling: Beginning again for fitness, average speeds question read on pls?

Two weeks ago I bought a new bike as I need to lose 20lb. Today I was able to go 23 miles but it took me three hours. I have not ridden a bike since I was 16, I am now 25. I was 45lb overweight, but I lost around half of it. Have 20 to go, this is when I bought the bike. I see no difference on the scales yet, as... show more Two weeks ago I bought a new bike as I need to lose 20lb. Today I was able to go 23 miles but it took me three hours. I have not ridden a bike since I was 16, I am now 25. I was 45lb overweight, but I lost around half of it. Have 20 to go, this is when I bought the bike. I see no difference on the scales yet, as I've only gone 85 miles so far since getting the bike. It is a mountain bike, which is heavier, so it is slower, should I have bought a road bike, I am riding it mainly on country roads, some of them are rough. My question is does this sound like I am riding fast enough to lose weight. I mean three hours, my knee started to hurt I could have gone another hour. According to some website I burned 1500 calories. Which is a start because I need to burn 75000 by december. The math there is 3300 calories x 20lb odd and the speed I ride will prob increase. I am sort of unfit. Any advice?
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