if you are getting that many calls preflop, you should be raising even more - raise to $4 or $5 there and get people outta there - the flop sucked for your hand and your good leadout bet was called by 3 others...when 3 players call a flush/straight draw heavy board, you need to be very wary - the turn card completed anyones flush draw or straight draw...I think leading is fine, and the $20 is a good amount (you still have the backdoor nut flush draw, slim but its something) - but betting the river is bad...your not beating anything thats calling you here -

rondavu is right - this is a glaring example of when the big pair needs to be given up -