How many weeks/months/years have you been riding? Sometimes it take a while if biking is new to you. Going out for a two hour ride for the first time is going to hurt anyone. Start by going out around 30 min or so to start and build up from there. It will take a while usually a few months to get in decent biking shape. Doing core training like yoga will also help improve your body for cycling. Good luck and keep at it...

BTW the frame size looks okay to me. Might want to play with the stem length/height to start if anything...